libweston/compositor: Do not map subsurfaces without buffer

We can end in `subsurface_committed()` in different scenarios
without the surface having an attached buffer. While setting
the mapped state to `true` in that case doesn't matter for
that (sub)surface itself, it triggers its own child subsurfaces
to get mapped when they shouldn't.

Closes #426



Signed-off-by: Robert Mader <robert.mader@collabora.com>