clients: A better fix for a crash when unlocking or unconfining a pointer

This is a rewrite of the fix in:
https://lists.freedesktop.org/archives/wayland-devel/2018-May/038140.html



It addresses Pekka's concerns about window getting destroyed before the
unlock/unconfine event is triggered.

Signed-off-by: Dima Ryazanov <dima@gmail.com>