message: Fix compilation error on GCC 4.8
diff: @@ -248,6 +248,7 @@ error stream_governor::confirm_downstream(const strong_actor_ptr& rebind_from,
  bool redeployable) {
  CAF_LOG_TRACE(CAF_ARG(rebind_from) << CAF_ARG(hdl)
  << CAF_ARG(initial_demand) << CAF_ARG(redeployable));
  + CAF_IGNORE_UNUSED(redeployable);
  auto path = local_subscribers_.find(rebind_from);
  if (path) {
  if (!local_subscribers_.confirm_path(rebind_from, hdl, initial_demand)) {
  @@ -363,6 +364,7 @@ error stream_governor::close_upstream(strong_actor_ptr& hdl) {
  void stream_governor::abort(strong_actor_ptr& hdl, const error& reason) {
  CAF_LOG_TRACE(CAF_ARG(hdl) << CAF_ARG(reason));
  + CAF_IGNORE_UNUSED(reason);
  if (local_subscribers_.remove_path(hdl))
  return;
  auto i = peers_.find(hdl);
  @@ -507,7 +509,7 @@ behavior core(stateful_actor<core_state>* self, filter_type initial_filter) {
  CAF_LOG_ERROR("Cannot join a data stream without downstream.");
  auto rp = self->make_response_promise();
  rp.deliver(sec::no_downstream_stages_defined);
  - return {stream_id{nullptr, 0}, nullptr};
  + return stream_type{stream_id{nullptr, 0}, nullptr};
  }
  auto next = stages.back();
  CAF_ASSERT(next != nullptr);
  