Merge branch 'rs/cocci-check-free-only-null'

A new coccinelle rule that catches a check of !pointer before the
pointer is free(3)d, which most likely is a bug.

* rs/cocci-check-free-only-null:
  cocci: detect useless free(3) calls

Author: gitster

contrib/coccinelle/free.cocci

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