libweston/input: Assert if we're still having a notify listener installed

Tracking correctly previous events shouldn't corrupt the surface destroy
signal list. This enforces that by ensuring that we wouldn't have
a .notify wl_listener still being set (which shouldn't happen if we do
eventually get a focus_in event that clears it out).

Suggested-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>