gdk: fix crash if visual is NULL
gdk: fix crash if visual is NULL

cairo_xlib_surface_get_visual might return NULL so we need to
check for NULL before trying to use it.

https://bugzilla.gnome.org/show_bug.cgi?id=752520

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