xen/blkback: don't call vbd_size() if bd_disk is NULL

...because vbd_size() dereferences bd_disk if bd_part is NULL.

Signed-off-by: Laszlo <Ersek&lt;lersek@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>