message: Detect nop in register_reading
diff: @@ -110,9 +110,10 @@ void multiplexer::register_reading(const socket_manager_ptr& mgr) {
  if (std::this_thread::get_id() == tid_) {
  if (mgr->mask() != operation::none) {
  CAF_ASSERT(index_of(mgr) != -1);
  - mgr->mask_add(operation::read);
  + if (mgr->mask_add(operation::read)) {
  auto& fd = pollset_[index_of(mgr)];
  fd.events |= input_mask;
  + }
  } else if (mgr->mask_add(operation::read)) {
  add(mgr);
  }
  