Fix leak of identifier in macro definition.

Fix leak of identifier in macro definition.

Valgrind detects leaks like the following (gdb.base/macscp.exp).
This patch fixes 1 of the 3 leaks (the last one in the list below).

The remaining leaks are better fixed in splay_tree_remove  and
splay_tree_insert in libiberty.
Tested on debian/amd64, natively and under valgrind.

==22285== 64 (48 direct, 16 indirect) bytes in 1 blocks are definitely lost in loss record 737 of 3,377
==22285==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==22285==    by 0x4049E7: xmalloc (common-utils.c:44)
==22285==    by 0x533A20: new_macro_key(macro_table*, char const*, macro_source_file*, int) (macrotab.c:355)
==22285==    by 0x53438B: macro_define_function(macro_source_file*, int, char const*, int, char const**, char const*) (macrotab.c:822)
==22285==    by 0x52F945: macro_define_command(char const*, int) (macrocmd.c:409)
...
==22285== 128 (96 direct, 32 indirect) bytes in 2 blocks are definitely lost in loss record 1,083 of 3,377
==22285==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==22285==    by 0x4049E7: xmalloc (common-utils.c:44)
==22285==    by 0x533A20: new_macro_key(macro_table*, char const*, macro_source_file*, int) (macrotab.c:355)
==22285==    by 0x534277: macro_define_object_internal(macro_source_file*, int, char const*, char const*, macro_special_kind) (macrotab.c:776)
==22285==    by 0x52F7E0: macro_define_command(char const*, int) (macrocmd.c:414)
...
==22285== 177 bytes in 19 blocks are definitely lost in loss record 1,193 of 3,377
==22285==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==22285==    by 0x4049E7: xmalloc (common-utils.c:44)
==22285==    by 0x52F5BD: extract_identifier(char const**, int) (macrocmd.c:316)
==22285==    by 0x52F77D: macro_define_command(char const*, int) (macrocmd.c:355)
gdb/ChangeLog
2019-02-06  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* macrocmd.c (extract_identifier): Return
	a gdb::unique_xmalloc_ptr<char> instead of a char *, and update
	callers.