xwm: Fix a weston crash when a window surface is created after unmap

xwm: Fix a weston crash when a window surface is created after unmap

If windows are created and quickly destroyed it's possible that they'll be
on the unpaired window list at the time of surface creation.  The surface
destroy listener for that surface isn't properly freed and a crash happens
some time later.

This patch removes the window from the unpaired list during unmap, so we
should never get to the destroy handler with a surface destroy listener set.

Just in case there's another path to that failure, I've also removed the
surface destroy listener in the destory handler.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Jasper St. Pierre <jstpierre@mecheye.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>