We shouldn't get here with RENAME_EXCHANGE set and no
target_ip, but let's be defensive, because xfs_cross_rename()
will dereference it.

Spotted by Coverity.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDave Chinner <david@fromorbit.com>