coccinelle: add a rule to make "type" code use FREE_AND_NULL()

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/free.cocci

@@ -9,3 +9,11 @@ expression E;
 @@
 - if (!E)
   free(E);
+
+@@
+type T;
+T *ptr;
+@@
+- free(ptr);
+- ptr = NULL;
++ FREE_AND_NULL(ptr);