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

clutter/x11/clutter-device-manager-xi2.c

@@ -774,7 +774,10 @@ clutter_device_manager_xi2_translate_event (ClutterEventTranslator *translator,
 
   event->any.stage = stage;
 
+  if (stage_x11 != NULL)
     window_scale = stage_x11->scale_factor;
+  else
+    window_scale = 1;
 
   switch (xi_event->evtype)
     {