nsswitch:krb5_plugin: Avoid a possible double free

nsswitch:krb5_plugin: Avoid a possible double free

Found by Covscan.

"Error: USE_AFTER_FREE (CWE-416):
samba-4.20.0rc2/nsswitch/krb5_plugin/winbind_krb5_localauth.c:122: freed_arg: ""wbcFreeMemory"" frees ""pwd"".
samba-4.20.0rc2/nsswitch/krb5_plugin/winbind_krb5_localauth.c:144: pass_freed_arg: Passing freed pointer ""pwd"" as an argument to ""wbcFreeMemory"".
  142|   		break;
  143|   	}
  144|-> 	wbcFreeMemory(pwd);
  145|   	if (code != 0) {
  146|   		goto out;"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Signed-off-by: Martin Schwenke <mschwenke@ddn.com>