Don't let total_queued go below zero.

main.c

@@ -1191,6 +1191,7 @@ static void inc_queued(void)
 static void dec_queued(void)
 {
 	pthread_mutex_lock(&qd_lock);
+	if (total_queued > 0)
 		total_queued--;
 	pthread_mutex_unlock(&qd_lock);
 	dec_staged(1);