[gdb] Fix heap-buffer-overflow in child_path

When compiling gdb with '-lasan -fsanitizer=address' and running tests with:
- export ASAN_OPTIONS="detect_leaks=0:alloc_dealloc_mismatch=0", and
- a target board using local-board.exp, which sets sysroot to ""
we run into a heap-buffer-overflow in child_path for f.i. gdb.arch/amd64-byte:
...
==3997==ERROR: AddressSanitizer: heap-buffer-overflow on address \
  0x60200002abcf at pc 0x5602acdf6872 bp 0x7ffe5237a090 sp 0x7ffe5237a080
READ of size 1 at 0x60200002abcf thread T0
    #0 0x5602acdf6871 in child_path(char const*, char const*) \
                      gdb/common/pathstuff.c:161
    riscvarchive#1 0x5602adb06587 in find_separate_debug_file gdb/symfile.c:1483
    riscvarchive#2 0x5602adb06f2f in find_separate_debug_file_by_debuglink[abi:cxx11](...) \
                      gdb/symfile.c:1563
    riscvarchive#3 0x5602ad13b743 in elf_symfile_read gdb/elfread.c:1293
    riscvarchive#4 0x5602adb01cfa in read_symbols gdb/symfile.c:798
    riscvarchive#5 0x5602adb03769 in syms_from_objfile_1 gdb/symfile.c:1000
    riscvarchive#6 0x5602adb039d0 in syms_from_objfile gdb/symfile.c:1017
    riscvarchive#7 0x5602adb04551 in symbol_file_add_with_addrs gdb/symfile.c:1124
    riscvarchive#8 0x5602adb04ebf in symbol_file_add_from_bfd(...) gdb/symfile.c:1204
    riscvarchive#9 0x5602ada5a78d in solib_read_symbols(...) gdb/solib.c:695
    riscvarchive#10 0x5602ada5bdae in solib_add(char const*, int, int) gdb/solib.c:1004
    riscvarchive#11 0x5602ada49bcd in enable_break gdb/solib-svr4.c:2394
    riscvarchive#12 0x5602ada4dae9 in svr4_solib_create_inferior_hook gdb/solib-svr4.c:3028
    riscvarchive#13 0x5602ada5d4f1 in solib_create_inferior_hook(int) gdb/solib.c:1215
    riscvarchive#14 0x5602ad347f66 in post_create_inferior(target_ops*, int) \
                          gdb/infcmd.c:467
    riscvarchive#15 0x5602ad348b3c in run_command_1 gdb/infcmd.c:663
    riscvarchive#16 0x5602ad348e55 in run_command gdb/infcmd.c:686
    riscvarchive#17 0x5602acd7d32b in do_const_cfunc gdb/cli/cli-decode.c:106
    riscvarchive#18 0x5602acd84bfe in cmd_func(cmd_list_element*, char const*, int) \
                          gdb/cli/cli-decode.c:1892
    riscvarchive#19 0x5602adc62a90 in execute_command(char const*, int) gdb/top.c:630
    riscvarchive#20 0x5602ad5053e6 in catch_command_errors gdb/main.c:372
    riscvarchive#21 0x5602ad507eb1 in captured_main_1 gdb/main.c:1138
    riscvarchive#22 0x5602ad5081ec in captured_main gdb/main.c:1163
    riscvarchive#23 0x5602ad508281 in gdb_main(captured_main_args*) gdb/main.c:1188
    riscvarchive#24 0x5602ac9ddc3a in main gdb/gdb.c:32
    riscvarchive#25 0x7f582b56eb96 in __libc_start_main \
                       (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    riscvarchive#26 0x5602ac9dda09 in _start \
                       (/home/smarchi/build/binutils-gdb/gdb/gdb+0x19a2a09)

0x60200002abcf is located 1 bytes to the left of 1-byte region \
  [0x60200002abd0,0x60200002abd1)
allocated by thread T0 here:
    #0 0x7f582e0e4b50 in __interceptor_malloc \
                      (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xdeb50)
    riscvarchive#1 0x5602acdd3656 in xmalloc gdb/common/common-utils.c:44
    riscvarchive#2 0x5602aefe17d1 in xstrdup libiberty/xstrdup.c:34
    riscvarchive#3 0x5602acdf61f6 in gdb_realpath(char const*) gdb/common/pathstuff.c:80
    riscvarchive#4 0x5602adb06278 in find_separate_debug_file gdb/symfile.c:1444
    riscvarchive#5 0x5602adb06f2f in find_separate_debug_file_by_debuglink[abi:cxx11](...) \
                      gdb/symfile.c:1563
    riscvarchive#6 0x5602ad13b743 in elf_symfile_read gdb/elfread.c:1293
    riscvarchive#7 0x5602adb01cfa in read_symbols gdb/symfile.c:798
    riscvarchive#8 0x5602adb03769 in syms_from_objfile_1 gdb/symfile.c:1000
    riscvarchive#9 0x5602adb039d0 in syms_from_objfile gdb/symfile.c:1017
    riscvarchive#10 0x5602adb04551 in symbol_file_add_with_addrs gdb/symfile.c:1124
    riscvarchive#11 0x5602adb04ebf in symbol_file_add_from_bfd(...) gdb/solib.c:695
    riscvarchive#13 0x5602ada5bdae in solib_add(char const*, int, int) gdb/solib.c:1004
    riscvarchive#14 0x5602ada49bcd in enable_break gdb/solib-svr4.c:2394
    riscvarchive#15 0x5602ada4dae9 in svr4_solib_create_inferior_hook gdb/solib-svr4.c:3028
    riscvarchive#16 0x5602ada5d4f1 in solib_create_inferior_hook(int) gdb/solib.c:1215
    riscvarchive#17 0x5602ad347f66 in post_create_inferior(target_ops*, int) \
                       gdb/infcmd.c:467
    riscvarchive#18 0x5602ad348b3c in run_command_1 gdb/infcmd.c:663
    riscvarchive#19 0x5602ad348e55 in run_command gdb/infcmd.c:686
    riscvarchive#20 0x5602acd7d32b in do_const_cfunc gdb/cli/cli-decode.c:106
    riscvarchive#21 0x5602acd84bfe in cmd_func(cmd_list_element*, char const*, int) \
                       gdb/cli/cli-decode.c:1892
    riscvarchive#22 0x5602adc62a90 in execute_command(char const*, int) gdb/top.c:630
    riscvarchive#23 0x5602ad5053e6 in catch_command_errors gdb/main.c:372
    riscvarchive#24 0x5602ad507eb1 in captured_main_1 gdb/main.c:1138
    riscvarchive#25 0x5602ad5081ec in captured_main gdb/main.c:1163
    riscvarchive#26 0x5602ad508281 in gdb_main(captured_main_args*) gdb/main.c:1188
    riscvarchive#27 0x5602ac9ddc3a in main gdb/gdb.c:32
    riscvarchive#28 0x7f582b56eb96 in __libc_start_main \
                       (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

SUMMARY: AddressSanitizer: heap-buffer-overflow gdb/common/pathstuff.c:161 \
  in child_path(char const*, char const*)
Shadow bytes around the buggy address:
  0x0c047fffd520: fa fa fd fd fa fa fd fd fa fa fd fa fa fa fd fa
  0x0c047fffd530: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fffd540: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fffd550: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fa
  0x0c047fffd560: fa fa fd fa fa fa fd fa fa fa fd fa fa fa 00 00
=>0x0c047fffd570: fa fa 07 fa fa fa 00 fa fa[fa]01 fa fa fa fa fa
  0x0c047fffd580: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffd590: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffd5a0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffd5b0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffd5c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==3997==ABORTING
...

The direct cause is that child_path gets called with parent == "", so this
test:
...
  if (IS_DIR_SEPARATOR (parent[parent_len - 1]))
...
accesses parent[-1].

[ There is an open discussion (1) about whether an empty sysroot should indeed
be represented internally as "".  But this patch focuses on fixing the
heap-buffer-overflow without any redesign. ]

Fix this by guarding the test with 'parent_len > 0'.

Note that the fix makes child_path behave the same for:
- parent == "/" && child == "/foo" (returns "foo")
- parent == "" and child == "/foo" (returns "foo").

Build and reg-tested on x86_64-linux.

(1) https://sourceware.org/ml/gdb-patches/2019-05/msg00193.html

gdb/ChangeLog:

2019-06-17  Tom de Vries  <[email protected]>

	PR gdb/24617
	* common/pathstuff.c (child_path): Make sure parent_len > 0 before
	accessing parent[parent_len - 1].

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-06-17  Tom de Vries  <[email protected]>
+
+	PR gdb/24617
+	* common/pathstuff.c (child_path): Make sure parent_len > 0 before
+	accessing parent[parent_len - 1].
+
 2019-06-17  Paul Pluzhnikov  <[email protected]>
 
 	PR gdb/24364

gdb/common/pathstuff.c

@@ -158,7 +158,7 @@ child_path (const char *parent, const char *child)
   /* The parent path must be a directory and the child must contain at
      least one component underneath the parent.  */
   const char *child_component;
-  if (IS_DIR_SEPARATOR (parent[parent_len - 1]))
+  if (parent_len > 0 && IS_DIR_SEPARATOR (parent[parent_len - 1]))
     {
       /* The parent path ends in a directory separator, so it is a
 	 directory.  The first child component starts after the common