ubi: Check the presence of volume before call ubi_fastmap_destroy_checkmap()

Else there may be oops when fastmap is enabled and init_volumes() fails.
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/vtbl.c

@@ -852,6 +852,8 @@ int ubi_read_volume_table(struct ubi_device *ubi, struct ubi_attach_info *ai)
 out_free:
 	vfree(ubi->vtbl);
 	for (i = 0; i < ubi->vtbl_slots + UBI_INT_VOL_COUNT; i++) {
+		if (!ubi->volumes[i])
+			continue;
 		ubi_fastmap_destroy_checkmap(ubi->volumes[i]);
 		kfree(ubi->volumes[i]);
 		ubi->volumes[i] = NULL;