message: Check for valid m_notify_state_change_fn during shutdown to avoid crash in case that this callback has not been set.
diff: @@ -2199,6 +2199,7 @@ void wamp_session::transition_to_closed()
  // would be a programming error.
  try {
  + if (m_notify_state_change_fn)
  m_notify_state_change_fn(*this, false);
  }
  catch (...) {
  