gdkwindow: Fix potential NULL pointer dereference
gdkwindow: Fix potential NULL pointer dereference

gdk_window_ensure_native() can end up with a NULL parent pointer, which
it passes to find_native_parent_above()…but that expects a non-NULL
parent.

Found with scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=712760

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