Fix gdb/python/python.c use-after-free

Fix gdb/python/python.c use-after-free

Valgrind shows:

 ==26964== Invalid read of size 1
 ==26964==    at 0x6E14100: __GI_strcmp (strcmp.S:180)
 ==26964==    by 0x6DB55AA: setlocale (setlocale.c:238)
 ==26964==    by 0x4E0455: _initialize_python() (python.c:1731)
 ==26964==    by 0x786731: initialize_all_files() (init.c:319)
 ==26964==    by 0x72EF0A: gdb_init(char*) (top.c:1929)
 ==26964==    by 0x60BCAC: captured_main(void*) (main.c:863)
 ==26964==    by 0x606AD5: catch_errors(int (*)(void*), void*, char*, return_mask) (exceptions.c:234)
 ==26964==    by 0x60C608: gdb_main(captured_main_args*) (main.c:1165)
 ==26964==    by 0x40CAEC: main (gdb.c:32)
 ==26964==  Address 0x81d30a0 is 0 bytes inside a block of size 181 free'd
 ==26964==    at 0x4C29CF0: free (vg_replace_malloc.c:530)
 ==26964==    by 0x6DB5B65: setname (setlocale.c:201)
 ==26964==    by 0x6DB5B65: setlocale (setlocale.c:388)
 ==26964==    by 0x4E037F: _initialize_python() (python.c:1712)
 ==26964==    by 0x786731: initialize_all_files() (init.c:319)
 ==26964==    by 0x72EF0A: gdb_init(char*) (top.c:1929)
 ==26964==    by 0x60BCAC: captured_main(void*) (main.c:863)
 ==26964==    by 0x606AD5: catch_errors(int (*)(void*), void*, char*, return_mask) (exceptions.c:234)
 ==26964==    by 0x60C608: gdb_main(captured_main_args*) (main.c:1165)
 ==26964==    by 0x40CAEC: main (gdb.c:32)

The problem is doing this:

  oldloc = setlocale (LC_ALL, NULL);
  setlocale (LC_ALL, "");
  ...
  setlocale (LC_ALL, oldloc);

I.e., the second setlocale call frees 'oldloc'.

From http://pubs.opengroup.org/onlinepubs/9699919799/functions/setlocale.html :

 "The returned string pointer might be invalidated or the string
 content might be overwritten by a subsequent call to setlocale()."

gdb/ChangeLog:
2016-05-03  Pedro Alves <palves@redhat.com>

	PR python/20037
	* python/python.c (_initialize_python) [IS_PY3K]: xstrdup/xfree
	oldloc.