keep the dog in the kennel unless there's a queue

As long as we are not queuing any threads, there is no queue to
move. We record the queue marker the first time we notice queuing and
only then see if it doesn't move.

Fixes #2794

bin/varnishd/cache/cache_wrk.c

@@ -510,14 +510,15 @@ pool_herder(void *priv)
 		 * Instead we implement a watchdog and kill the worker if
 		 * nothing has been dequeued for that long.
 		 */
-		if (dq != pp->ndequeued) {
+		if (pp->lqueue == 0) {
+			dq = pp->ndequeued + 1;
+		} else if (dq != pp->ndequeued) {
 			dq = pp->ndequeued;
 			dqt = VTIM_real();
-		} else if (pp->lqueue &&
-		    VTIM_real() - dqt > cache_param->wthread_watchdog) {
+		} else if (VTIM_real() - dqt > cache_param->wthread_watchdog) {
 			VSL(SLT_Error, 0,
-			   "Pool Herder: Queue does not move ql=%u dt=%f",
-			   pp->lqueue, VTIM_real() - dqt);
+			    "Pool Herder: Queue does not move ql=%u dt=%f",
+			    pp->lqueue, VTIM_real() - dqt);
 			WRONG("Worker Pool Queue does not move");
 		}
 		wthread_min = cache_param->wthread_min;