message: Add missing nullptr check
diff: @@ -66,10 +66,12 @@ void send_as(const Source& src, const Dest& dest, Ts&&... xs) {
  nullptr, std::forward<Ts>(xs)...);
  }
  -template <class Source, class Dest, class... Ts>
  +template <message_priority P = message_priority::normal, class Source,
  + class Dest, class... Ts>
  void unsafe_send_as(Source* src, const Dest& dest, Ts&&... xs) {
  - actor_cast<abstract_actor*>(dest)->eq_impl(make_message_id(), src->ctrl(),
  - src->context(),
  + if (dest)
  + actor_cast<abstract_actor*>(dest)->eq_impl(make_message_id(P),
  + src->ctrl(), src->context(),
  std::forward<Ts>(xs)...);
  }
  