Discard work that is stale in the get_queued() function, returning NULL instead.

sgminer.c

@@ -6327,9 +6327,15 @@ struct work *get_queued(struct cgpu_info *cgpu)
 	wr_lock(&cgpu->qlock);
 	if (cgpu->unqueued_work) {
 		work = cgpu->unqueued_work;
+		if (unlikely(stale_work(work, false))) {
+			discard_work(work);
+			work = NULL;
+			goto out_unlock;
+		}
 		__add_queued(cgpu, work);
 		cgpu->unqueued_work = NULL;
 	}
+out_unlock:
 	wr_unlock(&cgpu->qlock);
 
 	return work;