md: raid1: check rdev before reference in raid1_sync_request func

In raid1_sync_request func, rdev should be checked before reference.
Signed-off-by: NZhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: NSong Liu <songliubraving@fb.com>

drivers/md/raid1.c

@@ -2782,7 +2782,7 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 				write_targets++;
 			}
 		}
-		if (bio->bi_end_io) {
+		if (rdev && bio->bi_end_io) {
 			atomic_inc(&rdev->nr_pending);
 			bio->bi_iter.bi_sector = sector_nr + rdev->data_offset;
 			bio_set_dev(bio, rdev->bdev);