x11-selection: Plug potential GInputStream leak

The stream was not freed in the error case. I have never observed this
one in practice though.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1001

src/x11/meta-selection-source-x11.c

@@ -190,6 +190,7 @@ read_mimetypes_cb (GInputStream *stream,
     {
       g_task_return_error (task, error);
       g_object_unref (task);
+      g_object_unref (stream);
       return;
     }