Fix heap-buffer-overflow in explicit_location_lex_one

Fix heap-buffer-overflow in explicit_location_lex_one

I build GDB with -fsanitize=address, and see the error in tests,

(gdb) PASS: gdb.linespec/ls-errs.exp: lang=C++: break 3 foo
break -line 3 foo^M
=================================================================^M
==4401==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000047487 at pc 0x819d8e bp 0x7fff4e4e6bb0 sp 0x7fff4e4e6ba8^M
READ of size 1 at 0x603000047487 thread T0^[[1m^[[0m^M
    #0 0x819d8d in explicit_location_lex_one /home/yao/SourceCode/gnu/gdb/git/gdb/location.c:502^M
    #1 0x81a185 in string_to_explicit_location(char const**, language_defn const*, int) /home/yao/SourceCode/gnu/gdb/git/gdb/location.c:556^M
    #2 0x81ac10 in string_to_event_location(char**, language_defn const*) /home/yao/SourceCode/gnu/gdb/git/gdb/location.c:687^

the code in question is:

>         /* Special case: C++ operator,.  */
>         if (language->la_language == language_cplus
>             && strncmp (*inp, "operator", 8)  <--- [1]
>             && (*inp)[9] == ',')
>           (*inp) += 9;
>         ++(*inp);

The error is caused by the access to (*inp)[9] if 9 is out of its bounds.
However [1] looks odd to me, because if strncmp returns true (non-zero),
the following check "(*inp)[9] == ','" makes no sense any more.  I
suspect it was a typo in the code we meant to "strncmp () == 0".  Another
problem in the code above is that if *inp is "operator,", we first
increment *inp by 9, and then increment it by one again, which is wrong
to me.  We should only increment *inp by 8 to skip "operator", and go
back to the loop header to decide where we stop.

gdb:

2016-08-15  Yao Qi  <yao.qi@linaro.org>

	* location.c (explicit_location_lex_one): Compare the return
	value of strncmp with zero.  Don't check (*inp)[9].  Increment
	*inp by 8.