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

A follow-up to the existing "type" rule added in an earlier
change. This catches some occurrences that are missed by the previous
rule.

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

contrib/coccinelle/free.cocci

@@ -17,3 +17,10 @@ T *ptr;
 - free(ptr);
 - ptr = NULL;
 + FREE_AND_NULL(ptr);
+
+@@
+expression E;
+@@
+- free(E);
+- E = NULL;
++ FREE_AND_NULL(E);