Ensure that we never call a VDP with a zero length unless we are done.

Fixes #1561

bin/varnishd/cache/cache.h

@@ -833,6 +833,7 @@ VDP_bytes(struct req *req, enum vdp_action act, const void *ptr, ssize_t len)
 
 	CHECK_OBJ_NOTNULL(req, REQ_MAGIC);
 
+	assert(act > VDP_NULL || len > 0);
 	/* Call the present layer, while pointing to the next layer down */
 	i = req->vdp_nxt--;
 	assert(i >= 0 && i < N_VDPS);

bin/varnishd/cache/cache_obj.c

@@ -77,9 +77,12 @@ ObjIter(struct objiter *oi, void **p, ssize_t *l)
 			oi->st = VTAILQ_FIRST(&oi->obj->body->list);
 		else
 			oi->st = VTAILQ_NEXT(oi->st, list);
+		while(oi->st != NULL && oi->st->len == 0)
+			oi->st = VTAILQ_NEXT(oi->st, list);
 		if (oi->st != NULL) {
 			*p = oi->st->ptr;
 			*l = oi->st->len;
+			assert(*l > 0);
 			return (OIS_DATA);
 		}
 		return (OIS_DONE);