Don't call a zero byte response a "Good Recv".

git-svn-id: http://www.varnish-cache.org/svn/trunk@3111 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache_backend_poll.c

@@ -221,6 +221,9 @@ vbp_poke(struct vbp_target *vt)
 		return;
 	}
 
+	if (rlen == 0)
+		return;
+
 	/* So we have a good receive ... */
 	t_now = TIM_real();
 	vt->last = t_now - t_start;