[readline] Fix heap-buffer-overflow in update_line

When:
- building trunk gdb with '-fsanitize=address -lasan',
- running gdb tests with "export ASAN_OPTIONS=detect_leaks=0",
I run into a heap-buffer-overflow failure for
gdb.base/utf8-identifiers.exp.

In more detail, the libasan error report looks like this:
...
=================================================================
==22340==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x619000054a80 at pc 0x7fcd0306b4c9 bp 0x7fffb1a8d880 sp 0x7fffb1a8d030
READ of size 32766 at 0x619000054a80 thread T0
    #0 0x7fcd0306b4c8  (/usr/lib64/libasan.so.4+0xae4c8)
    #1 0x15f12a1 in update_line
/data/gdb_versions/devel/src/readline/display.c:1377
    #2 0x15f03cb in rl_redisplay
/data/gdb_versions/devel/src/readline/display.c:1204
    #3 0x15bf932 in readline_internal_setup
/data/gdb_versions/devel/src/readline/readline.c:394
    #4 0x15fe723 in _rl_callback_newline
/data/gdb_versions/devel/src/readline/callback.c:89
    #5 0x15fe7ef in rl_callback_handler_install
/data/gdb_versions/devel/src/readline/callback.c:102
    #6 0xd7bce6 in gdb_rl_callback_handler_install(char const*)
/data/gdb_versions/devel/src/gdb/event-top.c:319
    #7 0xd7c0c6 in display_gdb_prompt(char const*)
/data/gdb_versions/devel/src/gdb/event-top.c:409
    #8 0xd7d6c1 in command_line_handler(std::unique_ptr<char,
gdb::xfree_deleter<char> >&&)
/data/gdb_versions/devel/src/gdb/event-top.c:776
    #9 0xd7b92a in gdb_rl_callback_handler
/data/gdb_versions/devel/src/gdb/event-top.c:217
    #10 0x15ff479 in rl_callback_read_char
/data/gdb_versions/devel/src/readline/callback.c:220
    #11 0xd7b4d5 in gdb_rl_callback_read_char_wrapper_noexcept
/data/gdb_versions/devel/src/gdb/event-top.c:175
    #12 0xd7b6b5 in gdb_rl_callback_read_char_wrapper
/data/gdb_versions/devel/src/gdb/event-top.c:192
    #13 0xd7c8aa in stdin_event_handler(int, void*)
/data/gdb_versions/devel/src/gdb/event-top.c:514
    #14 0xd76ca7 in handle_file_event
/data/gdb_versions/devel/src/gdb/event-loop.c:731
    #15 0xd7751f in gdb_wait_for_event
/data/gdb_versions/devel/src/gdb/event-loop.c:857
    #16 0xd7547e in gdb_do_one_event()
/data/gdb_versions/devel/src/gdb/event-loop.c:321
    #17 0xd75526 in start_event_loop()
/data/gdb_versions/devel/src/gdb/event-loop.c:370
    #18 0x101b04c in captured_command_loop
/data/gdb_versions/devel/src/gdb/main.c:331
    #19 0x101de73 in captured_main
/data/gdb_versions/devel/src/gdb/main.c:1173
    #20 0x101df03 in gdb_main(captured_main_args*)
/data/gdb_versions/devel/src/gdb/main.c:1188
    #21 0x872dba in main /data/gdb_versions/devel/src/gdb/gdb.c:32
    #22 0x7fcd00f2ff49 in __libc_start_main (/lib64/libc.so.6+0x20f49)
    #23 0x872bc9 in _start (/data/gdb_versions/devel/build/gdb/gdb+0x872bc9)

0x619000054a80 is located 0 bytes to the right of 1024-byte region
[0x619000054680,0x619000054a80)
allocated by thread T0 here:
    #0 0x7fcd03099510 in malloc (/usr/lib64/libasan.so.4+0xdc510)
    #1 0xae0078 in xmalloc
/data/gdb_versions/devel/src/gdb/common/common-utils.c:44
    #2 0x15eaccb in init_line_structures
/data/gdb_versions/devel/src/readline/display.c:458
    #3 0x15eb4d8 in rl_redisplay
/data/gdb_versions/devel/src/readline/display.c:526
    #4 0x15bf932 in readline_internal_setup
/data/gdb_versions/devel/src/readline/readline.c:394
    #5 0x15fe723 in _rl_callback_newline
/data/gdb_versions/devel/src/readline/callback.c:89
    #6 0x15fe7ef in rl_callback_handler_install
/data/gdb_versions/devel/src/readline/callback.c:102
    #7 0xd7bce6 in gdb_rl_callback_handler_install(char const*)
/data/gdb_versions/devel/src/gdb/event-top.c:319
    #8 0xd7c0c6 in display_gdb_prompt(char const*)
/data/gdb_versions/devel/src/gdb/event-top.c:409
    #9 0xaa041b in cli_interp_base::pre_command_loop()
/data/gdb_versions/devel/src/gdb/cli/cli-interp.c:286
    #10 0xf5342a in interp_pre_command_loop(interp*)
/data/gdb_versions/devel/src/gdb/interps.c:320
    #11 0x101b047 in captured_command_loop
/data/gdb_versions/devel/src/gdb/main.c:328
    #12 0x101de73 in captured_main
/data/gdb_versions/devel/src/gdb/main.c:1173
    #13 0x101df03 in gdb_main(captured_main_args*)
/data/gdb_versions/devel/src/gdb/main.c:1188
    #14 0x872dba in main /data/gdb_versions/devel/src/gdb/gdb.c:32
    #15 0x7fcd00f2ff49 in __libc_start_main (/lib64/libc.so.6+0x20f49)

SUMMARY: AddressSanitizer: heap-buffer-overflow
(/usr/lib64/libasan.so.4+0xae4c8)
Shadow bytes around the buggy address:
  0x0c3280002900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c3280002910: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c3280002920: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c3280002930: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c3280002940: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c3280002950:[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c3280002960: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c3280002970: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c3280002980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c3280002990: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c32800029a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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
==22340==ABORTING
...

I've written an assert in rl_redisplay that formulates the error condition:
...
@@ -1387,6 +1389,10 @@ rl_redisplay (void)
          cpos_adjusted = 0;
+         assert (last_lmargin + (_rl_screenwidth + visible_wrap_offset)
+                 <= line_size);
+         assert (lmargin + (_rl_screenwidth + (lmargin ? 0 : wrap_offset))
+                 <= line_size);
          update_line (&visible_line[last_lmargin],
                       &invisible_line[lmargin],
                       0,
                       _rl_screenwidth + visible_wrap_offset,
                       _rl_screenwidth + (lmargin ? 0 : wrap_offset),
                       0);
...
which triggers without needing the address sanitizer (or even an executable),
like this:
...
$ TERM=dumb gdb -q -ex "set width 0"
gdb: src/display.c:1393: rl_redisplay: Assertion
`last_lmargin + (_rl_screenwidth + visible_wrap_offset) <= line_size'
failed.
Aborted (core dumped)
...

The basic problem is this: visible_line and invisible_line have length
line_size, but the update_line call assumes that line_size is at least
_rl_screenwidth + 1.  Executing "set width 0" sets _rl_screenwidth to 32766 but
doesn't affect line_size, which is initialized to 1024.

Fix this by ensuring in init_line_structures and rl_redisplay that line_size
is at least _rl_screenwidth + 1.

Tested on x86_64-linux.

Reviewed by readline maintainer (
https://sourceware.org/ml/gdb-patches/2019-05/msg00566.html ).

readline/ChangeLog.gdb:

2019-07-12  Tom de Vries  <[email protected]>
	    Chet Ramey  <[email protected]>

	PR cli/24514
	* readline/display.c (init_line_structures, rl_redisplay): Ensure
	line_size is at	least _rl_screenwidth + 1.

readline/ChangeLog.gdb

@@ -1,3 +1,10 @@
+2019-07-12  Tom de Vries  <[email protected]>
+	    Chet Ramey  <[email protected]>
+
+	PR cli/24514
+	* readline/display.c (init_line_structures, rl_redisplay): Ensure
+	line_size is at	least _rl_screenwidth + 1.
+
 2019-01-31  Alan Hayward  <[email protected]>
 
 	* config.h.in: Add SOURCE/EXTENSION macros.

readline/display.c

@@ -450,6 +450,9 @@ init_line_structures (minsize)
 {
   register int n;
 
+  if (minsize <= _rl_screenwidth)	/* XXX - for gdb */
+    minsize = _rl_screenwidth + 1;
+
   if (invisible_line == 0)	/* initialize it */
     {
       if (line_size < minsize)
@@ -526,6 +529,8 @@ rl_redisplay ()
       init_line_structures (0);
       rl_on_new_line ();
     }
+  else if (line_size <= _rl_screenwidth)
+    init_line_structures (_rl_screenwidth + 1);
 
   /* Draw the line into the buffer. */
   cpos_buffer_position = -1;