GtkApplication: Prevent more crashes around shutdown
GtkApplication: Prevent more crashes around shutdown

gtk_application_shutdown clears the impl member, so all
callbacks, signal handlers, etc that might still be triggered
between a shutdown call and the return from the mainloop
better be prepared to deal with impl being NULL.

jigpu/gtk,grubersjoe/adwaita,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Adamovskiy/gtk,davidgumberg/gtk,grubersjoe/adwaita,Adamovskiy/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,chergert/gtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,chergert/gtk,Adamovskiy/gtk,jadahl/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk