cocci: FREE_AND_NULL(E) is safe to call on NULL

Just like we allow calling free(E) without checking if E is not
NULL, it is safe to call FREE_AND_NULL(E) unconditionally.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

tools/coccinelle/free.cocci

@@ -6,6 +6,8 @@ expression E;
   free(E);
 |
   commit_list_free(E);
+|
+  FREE_AND_NULL(E);
 )
 
 @@