message: Check for non-null in send_exit
diff: @@ -173,6 +173,7 @@ public:
  /// Sends an exit message to `dest`.
  template <class ActorHandle>
  void send_exit(const ActorHandle& dest, error reason) {
  + if (dest)
  dest->eq_impl(make_message_id(), ctrl(), context(),
  exit_msg{address(), std::move(reason)});
  }
  