Most of the callers are just calling ASSERT(!xfs_buf_geterror())
which means they are checking for bp->b_error == 0. If bp is null in
this case, we will assert fail, and hence it's no different in
result to oopsing because of a null bp. In some cases, errors have
already been checked for or the function returning the buffer can't
return a buffer with an error, so it's just a redundant assert.
Either way, the assert can either be removed.

The other two non-assert callers can just test for a buffer and
error properly.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>