xfs: fix btree scrub checking with regards to root-in-inode

In xchk_btree_check_owner, we can be passed a null buffer pointer.  This
should only happen for the root of a root-in-inode btree type, but we
should program defensively in case the btree cursor state ever gets
screwed up and we get a null buffer anyway.

Coverity-id: 1438713
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>