message: Delegate stream_msg::abort to the policies
diff: @@ -96,12 +96,13 @@ auto stream_msg_visitor::operator()(stream_msg::close&) -> result_type {
  auto stream_msg_visitor::operator()(stream_msg::abort& x) -> result_type {
  CAF_LOG_TRACE(CAF_ARG(x));
  - if (i_ == e_) {
  + if (i_ != e_ && self_->current_sender() != nullptr) {
  + i_->second->abort(self_->current_sender(), x.reason);
  + return {std::move(x.reason), i_};
  + }
  CAF_LOG_DEBUG("received stream_msg::abort for unknown stream");
  return {sec::unexpected_message, e_};
  }
  - return {std::move(x.reason), i_};
  -}
  auto stream_msg_visitor::operator()(stream_msg::ack_open& x) -> result_type {
  CAF_LOG_TRACE(CAF_ARG(x));
  