Zero all pthread_t identities upon cancelling threads.

util.c

@@ -680,8 +680,11 @@ void thr_info_cancel(struct thr_info *thr)
 
 	if (thr->q)
 		tq_freeze(thr->q);
-	if (pthread_cancel(thr->pth))
-		pthread_join(thr->pth, NULL);
+	if (thr->pth) {
+			if (pthread_cancel(thr->pth))
+				pthread_join(thr->pth, NULL);
+			thr->pth = 0L;
+	}
 }
 
 bool get_dondata(char **url, char **userpass)