cocci: Masquerade foreach loops as if statements

Inside the loop the element is in fact, not null. Only one occurrence
was caught by check_obj.cocci, which shows that more code is still not
understood by Coccinelle. For example, it doesn't seem to enjoy #undef
directives inside functions.

bin/varnishd/hash/hash_critbit.c

@@ -320,7 +320,7 @@ hcb_cleaner(struct worker *wrk, void *priv)
 			FREE_OBJ(y);
 		}
 		VTAILQ_FOREACH_SAFE(oh, &dead_h, hoh_list, oh2) {
-			CHECK_OBJ_NOTNULL(oh, OBJHEAD_MAGIC);
+			CHECK_OBJ(oh, OBJHEAD_MAGIC);
 			VTAILQ_REMOVE(&dead_h, oh, hoh_list);
 			HSH_DeleteObjHead(wrk, oh);
 		}

tools/coccinelle/vdef.h

@@ -24,6 +24,10 @@
 #define VTAILQ_HEAD_INITIALIZER(t)	0
 #define VSTAILQ_HEAD_INITIALIZER(t)	0
 
+#define VTAILQ_FOREACH(o, h, l)			if (o != NULL)
+#define VTAILQ_FOREACH_SAFE(o, h, l, o2)	if (o != NULL)
+#define VTAILQ_FOREACH_REVERSE(o, h, t, l)	if (o != NULL)
+
 /* include/vtree.h */
 #define VRBT_ENTRY(x)		unsigned
 #define VRBT_HEAD(x, y)		unsigned