Merge branch 'ep/maint-equals-null-cocci'

Introduce and apply coccinelle rule to discourage an explicit
comparison between a pointer and NULL, and applies the clean-up to
the maintenance track.

* ep/maint-equals-null-cocci:
  tree-wide: apply equals-null.cocci
  tree-wide: apply equals-null.cocci
  contrib/coccinnelle: add equals-null.cocci

wrapper.c

@@ -393,7 +393,7 @@ FILE *xfopen(const char *path, const char *mode)
 FILE *xfdopen(int fd, const char *mode)
 {
 	FILE *stream = fdopen(fd, mode);
-	if (stream == NULL)
+	if (!stream)
 		die_errno("Out of memory? fdopen failed");
 	return stream;
 }