sound-player: Fix invalid write after playback is cancelled early

The cancellable of a request might already be cancelled by the time
the cancelled_cb is connected resulting in finish_cb being called via
ca_context_cancel before g_cancellable_connect returns. In this case
the request that is written to has already been freed.

Fixes https://gitlab.gnome.org/GNOME/mutter/issues/1060

src/core/meta-sound-player.c

@@ -140,9 +140,11 @@ play_sound (MetaPlayRequest *req,
 
   if (req->cancellable)
     {
-      req->cancel_id =
+      gulong cancel_id =
         g_cancellable_connect (req->cancellable,
                                G_CALLBACK (cancelled_cb), req, NULL);
+      if (cancel_id)
+        req->cancel_id = cancel_id;
     }
 }