md-cluster: Fix potential error pointer dereference in resize_bitmaps()

The error handling calls md_bitmap_free(bitmap) which checks for NULL
but will Oops if we pass an error pointer.  Let's set "bitmap" to NULL
on this error path.

Fixes: afd75628 ("md-cluster/raid10: resize all the bitmaps before start reshape")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/md-cluster.c

@@ -1139,6 +1139,7 @@ static int resize_bitmaps(struct mddev *mddev, sector_t newsize, sector_t oldsiz
 		bitmap = get_bitmap_from_slot(mddev, i);
 		if (IS_ERR(bitmap)) {
 			pr_err("can't get bitmap from slot %d\n", i);
+			bitmap = NULL;
 			goto out;
 		}
 		counts = &bitmap->counts;