xfs: fix uninitialized variable in xfs_attr3_leaf_inactive

Dan Carpenter pointed out that error is uninitialized.  While there
never should be an attr leaf block with zero entries, let's not leave
that logic bomb there.

Fixes: 0bb9d159

 ("xfs: streamline xfs_attr3_leaf_inactive")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>