x11: Ensure we have a stage before accessing its fields

For some XI2 we do not have a Stage associated to the event window.

Original patch by: Giovanni Campagna <scampa.giovanni@gmail.com>
Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=708439
Release Clutter 1.15.96 (snapshot)
Post-release version bump to 1.15.97