Bugfix: When disabling device, ensure its prev_work (if any) gets freed properly

miner.c

@@ -6626,6 +6626,8 @@ void mt_disable_start(struct thr_info *mythr)
 	int thr_id = mythr->id;
 	
 	hashmeter2(mythr);
+	if (mythr->prev_work)
+		free_work(mythr->prev_work);
 	mythr->prev_work = mythr->work;
 	mythr->work = NULL;
 	mythr->_job_transition_in_progress = false;