a11y: handle atk race condition where widget has been destroyed
a11y: handle atk race condition where widget has been destroyed

If the widget has been destroyed since a DBus message had been sent,
we could be in a condition that the widget pointer exists but it does
not have a window.

This bails as if the widget didn't exist if there is no available
GdkWindow.

We also set the extents to 0 to be defensive since this is a vfunc
implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=746586

ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,jigpu/gtk,Adamovskiy/gtk,Lyude/gtk-,jigpu/gtk,jadahl/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,grubersjoe/adwaita,davidgumberg/gtk,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,Lyude/gtk-,jadahl/gtk,Lyude/gtk-,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,ahodesuka/gtk,ahodesuka/gtk,jadahl/gtk,chergert/gtk,Adamovskiy/gtk,Lyude/gtk-,jigpu/gtk,alexlarsson/gtk,jigpu/gtk,Adamovskiy/gtk,davidgumberg/gtk,davidgumberg/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,chergert/gtk,jigpu/gtk,ahodesuka/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,Lyude/gtk-,grubersjoe/adwaita,grubersjoe/adwaita,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk