shm: wl_shm_buffer_get_data() requires a valid pool.

There's no situation where a shm buffer without a pool makes sense,
so we enforce the pool's existence a little more rigidly.

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>