widget: Use an unchecked cast in _get_native()

widget: Use an unchecked cast in _get_native()

This is either no chagne because we know for a fact that the returned
value is a GtkNative - after all thats's the type we pass to
gtk_widget_get_ancestor().

Or it is a bug fix since casting NULL to a GtkNative using GTK_NATIVE()
is not going to work, but the API contract of gtk_widget_get_native()
explicitly allows a NULL return value.