cocci: detect useless free(3) calls

Add a semantic patch for removing checks that cause free(3) to only be
called with a NULL pointer, as that must be a programming mistake.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/free.cocci

@@ -3,3 +3,9 @@ expression E;
 @@
 - if (E)
   free(E);
+
+@@
+expression E;
+@@
+- if (!E)
+  free(E);