libxfs: don't send null bp to xfs_trans_brelse()

libxfs: don't send null bp to xfs_trans_brelse()

In this case, if bp is null, error is set, and we send
bp to xfs_trans_brelse, which will try to dereference it.

Test whether we actualy have a buffer before we try to
free it.

Same fix as was sent for kernelspace.

Coverity spotted this.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>