core: Fix memory selection source leak after clipboard owner disappears

When an app disappears after some data from it has been copied to the
clipboard, the owner of the clipboard selection becomes a new memory
selection source. The initial reference this new selection source is
never unref'ed, which leads to this being leaked on the next clipboard
selection owner change.

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

(cherry picked from commit a031ac067e)

src/core/meta-clipboard-manager.c

@@ -152,6 +152,7 @@ owner_changed_cb (MetaSelection       *selection,
                                                     display->saved_clipboard);
       g_set_object (&display->selection_source, new_owner);
       meta_selection_set_owner (selection, selection_type, new_owner);
+      g_object_unref (new_owner);
     }
 }