message: Fix invalid iterator bug in response multiplexing
diff: @@ -476,13 +476,14 @@ invoke_message_result scheduled_actor::consume(mailbox_element& x) {
  // neither awaited nor multiplexed, probably an expired timeout
  if (mrh == multiplexed_responses_.end())
  return im_dropped;
  - if (!invoke(this, mrh->second, x)) {
  + auto bhvr = std::move(mrh->second);
  + multiplexed_responses_.erase(mrh);
  + if (!invoke(this, bhvr, x)) {
  // try again with error if first attempt failed
  auto msg = make_message(make_error(sec::unexpected_response,
  x.move_content_to_message()));
  - mrh->second(msg);
  + bhvr(msg);
  }
  - multiplexed_responses_.erase(mrh);
  return im_success;
  }
  // Dispatch on the content of x.
  