xfs_db: don't crash in ablock if there's no inode

Make sure we actually have an inode selected before trying to unwrap its
attribute fork.  Found via a crash in xfs/288 with project quotas
enabled.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>