core: Free clipboard selection source on shutdown

The clipboard manager is the only code to ever set the display selection
source, so it should also be responsible for unsetting it when the
clipboard manager gets shut down.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1293

src/core/meta-clipboard-manager.c

@@ -171,6 +171,7 @@ meta_clipboard_manager_shutdown (MetaDisplay *display)
 {
   MetaSelection *selection;
 
+  g_clear_object (&display->selection_source);
   g_clear_pointer (&display->saved_clipboard, g_bytes_unref);
   g_clear_pointer (&display->saved_clipboard_mimetype, g_free);
   selection = meta_display_get_selection (display);