null-terminate string in linespec_location_completer

If I build gdb with -fsanitize=address and run tests, I get error,

malformed linespec error: unexpected colon^M
(gdb) PASS: gdb.linespec/ls-errs.exp: lang=C: break     :
break   :=================================================================^M
==3266==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000051451 at pc 0x2b5797a972a8 bp 0x7fffd8e0f3c0 sp 0x7fffd8e0f398^M
READ of size 2 at 0x602000051451 thread T0
    #0 0x2b5797a972a7 in __interceptor_strlen (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x322a7)^M
    tromey#1 0x7bd004 in compare_filenames_for_search(char const*, char const*) /home/yao/SourceCode/gnu/gdb/git/gdb/symtab.c:316^M
    tromey#2 0x7bd310 in iterate_over_some_symtabs(char const*, char const*, int (*)(symtab*, void*), void*, compunit_symtab*, compunit_symtab*) /home/yao/SourceCode/gnu/gdb/git/gdb/symtab.c:411^M
    tromey#3 0x7bd775 in iterate_over_symtabs(char const*, int (*)(symtab*, void*), void*) /home/yao/SourceCode/gnu/gdb/git/gdb/symtab.c:481^M
    tromey#4 0x7bda15 in lookup_symtab(char const*) /home/yao/SourceCode/gnu/gdb/git/gdb/symtab.c:527^M
    tromey#5 0x7d5e2a in make_file_symbol_completion_list_1 /home/yao/SourceCode/gnu/gdb/git/gdb/symtab.c:5635^M
    tromey#6 0x7d61e1 in make_file_symbol_completion_list(char const*, char const*, char const*) /home/yao/SourceCode/gnu/gdb/git/gdb/symtab.c:5684^M
    tromey#7 0x88dc06 in linespec_location_completer /home/yao/SourceCode/gnu/gdb/git/gdb/completer.c:288
....
0x602000051451 is located 0 bytes to the right of 1-byte region [0x602000051450,0x602000051451)^M
mallocated by thread T0 here:
    #0 0x2b5797ab97ef in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x547ef)^M
    tromey#1 0xbbfb8d in xmalloc /home/yao/SourceCode/gnu/gdb/git/gdb/common/common-utils.c:43^M
    tromey#2 0x88dabd in linespec_location_completer /home/yao/SourceCode/gnu/gdb/git/gdb/completer.c:273^M
    tromey#3 0x88e5ef in location_completer(cmd_list_element*, char const*, char const*) /home/yao/SourceCode/gnu/gdb/git/gdb/completer.c:531^M
    tromey#4 0x8902e7 in complete_line_internal /home/yao/SourceCode/gnu/gdb/git/gdb/completer.c:964^

The code in question is here

       file_to_match = (char *) xmalloc (colon - text + 1);
       strncpy (file_to_match, text, colon - text + 1);

it is likely that file_to_match is not null-terminated.  The patch is
to strncpy 'colon - text' bytes and explicitly set '\0'.

gdb:

2016-08-19  Yao Qi  <[email protected]>

	* completer.c (linespec_location_completer): Make file_to_match
	null-terminated.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2016-08-19  Yao Qi  <[email protected]>
+
+	* completer.c (linespec_location_completer): Make file_to_match
+	null-terminated.
+
 2016-08-19  Pedro Alves  <[email protected]>
 
 	* amd64-tdep.c (amd64_relocate_instruction) <callq>: Handle return

gdb/completer.c

@@ -264,7 +264,8 @@ linespec_location_completer (struct cmd_list_element *ignore,
       char *s;
 
       file_to_match = (char *) xmalloc (colon - text + 1);
-      strncpy (file_to_match, text, colon - text + 1);
+      strncpy (file_to_match, text, colon - text);
+      file_to_match[colon - text] = '\0';
       /* Remove trailing colons and quotes from the file name.  */
       for (s = file_to_match + (colon - text);
 	   s > file_to_match;