compositor: buffer can be non-NULL only if newly_attached

compositor: buffer can be non-NULL only if newly_attached

There is no need for weston_subsurface_commit_to_cache() to leave the
pending.buffer set. Reset it to NULL.

This makes pending.buffer always NULL if pending.newly_attached == 0.
IOW, pending.buffer cannot be non-NULL unless
pending.newly_attached == 1.

Therefore no need to check pending.buffer nor cached.buffer_ref.buffer
for the weston_surface_attach() calls.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>