[window] Fix setting small-screen before map
[window] Fix setting small-screen before map

If you set small-screen mode before mapping, this would cause an abort.
Fixed by checking the window actually exists before doing anything with
it.

jonnylamb/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,pwithnall/mx