MD: fix a null dereference

rdev->mddev could be null in start time.
Reported-by: Ming Lei <ming.lei@redhat.com>
Fix: 5a85071c(md: use a separate bio_set for synchronous IO.)
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -205,7 +205,7 @@ EXPORT_SYMBOL_GPL(bio_alloc_mddev);
 
 static struct bio *md_bio_alloc_sync(struct mddev *mddev)
 {
-	if (!mddev->sync_set)
+	if (!mddev || !mddev->sync_set)
 		return bio_alloc(GFP_NOIO, 1);
 
 	return bio_alloc_bioset(GFP_NOIO, 1, mddev->sync_set);