Prevent segfault on exit for when accessory threads don't exist.

main.c

@@ -1753,6 +1753,7 @@ void kill_work(void)
 
 	/* Kill the watchdog thread */
 	thr = &thr_info[watchdog_thr_id];
+	if (thr->pth)
 		pthread_cancel(*thr->pth);
 
 	/* Stop the mining threads*/
@@ -1767,8 +1768,10 @@ void kill_work(void)
 
 	/* Stop the others */
 	thr = &thr_info[stage_thr_id];
+	if (thr->pth)
 		pthread_cancel(*thr->pth);
 	thr = &thr_info[longpoll_thr_id];
+	if (thr->pth)
 		pthread_cancel(*thr->pth);
 
 	wc = calloc(1, sizeof(*wc));