gdbus: Fix removal of filter after last filter_data

gdbus: Fix removal of filter after last filter_data

If there's a signal watch that's also watching for name
(data->name_watch) currently we are trying to remove the message_filter
twice since we may have the following call chain:

filter_data_remove_callback()
  filter_data_free()
    g_dbus_remove_watch()
      filter_data_remove_callback()
	filter_data_free()
        dbus_connection_remove_filter()
  dbus_connection_remove_filter()

Because of this we can't currently watch for signals passing the bus
name. After this patch we don't have this issue anymore.

We fix it by removing the filter before calling filter_data_free() if we
are the last filter_data and thus avoid calling
dbus_connection_remove_filter() twice.