xfs: don't ASSERT on short form btree root pointer of zero

Don't ASSERT if the short form btree root pointer is zero.  Now that we
use xfs_verify_agbno to check all short form btree pointers, we'll let
that log the error and pass it to the upper layers.

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