blk-throttle: Move the list operation after list validation

We should move the list operation after validation.
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -423,12 +423,13 @@ static void throtl_qnode_add_bio(struct bio *bio, struct throtl_qnode *qn,
  */
 static struct bio *throtl_peek_queued(struct list_head *queued)
 {
-	struct throtl_qnode *qn = list_first_entry(queued, struct throtl_qnode, node);
+	struct throtl_qnode *qn;
 	struct bio *bio;
 
 	if (list_empty(queued))
 		return NULL;
 
+	qn = list_first_entry(queued, struct throtl_qnode, node);
 	bio = bio_list_peek(&qn->bios);
 	WARN_ON_ONCE(!bio);
 	return bio;
@@ -451,12 +452,13 @@ static struct bio *throtl_peek_queued(struct list_head *queued)
 static struct bio *throtl_pop_queued(struct list_head *queued,
 				     struct throtl_grp **tg_to_put)
 {
-	struct throtl_qnode *qn = list_first_entry(queued, struct throtl_qnode, node);
+	struct throtl_qnode *qn;
 	struct bio *bio;
 
 	if (list_empty(queued))
 		return NULL;
 
+	qn = list_first_entry(queued, struct throtl_qnode, node);
 	bio = bio_list_pop(&qn->bios);
 	WARN_ON_ONCE(!bio);