compositor: access correct weston_buffer union field according to type

Stop writing weston_buffer::shm_buffer when weston_buffer::type is not
WESTON_BUFFER_SHM. Instead, explicitly write to the union field
that corresponds to the buffer type.
Also add a comment why we clear the shm_buffer/dmabuf/legacy_buffer
pointer here.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>