xfs: don't assert when on-disk btree pointers are garbage

Don't ASSERT when we encounter bad on-disk btree pointers in the debug
check functions.  Log the error to leave breadcrumbs and let the upper
layers deal with it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>