onscreen/native: Forget the view pointer when it is detached

Otherwise it would linger after a hotplug event, pointing to the old
(destroyed) view. And it's much easier to debug a NULL pointer than
a pointer to freed memory.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3118>

src/backends/native/meta-onscreen-native.c

@@ -2483,4 +2483,5 @@ void
 meta_onscreen_native_detach (MetaOnscreenNative *onscreen_native)
 {
   clear_invalidation_handlers (onscreen_native);
+  onscreen_native->view = NULL;
 }