Btrfs: fix oops on NULL trans handle in btrfs_truncate

If we fail to reserve space in the transaction during truncate, we can
error out with a NULL trans handle.  The cleanup code needs an extra
check to make sure we aren't trying to use the bad handle.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/inode.c

@@ -6529,6 +6529,7 @@ end_trans:
 		ret = btrfs_orphan_del(NULL, inode);
 	}
 
+	if (trans) {
 		trans->block_rsv = &root->fs_info->trans_block_rsv;
 		ret = btrfs_update_inode(trans, root, inode);
 		if (ret && !err)
@@ -6537,6 +6538,7 @@ end_trans:
 		nr = trans->blocks_used;
 		ret = btrfs_end_transaction_throttle(trans, root);
 		btrfs_btree_balance_dirty(root, nr);
+	}
 
 out:
 	btrfs_free_block_rsv(root, rsv);