message: Fix system discovery
diff: @@ -300,13 +300,10 @@ void MavsdkImpl::receive_message(mavlink_message_t& message, Connection* connect
  std::lock_guard<std::recursive_mutex> lock(_systems_mutex);
  - // The only situation where we create a system with sysid 0 is when we initialize the connection
  - // to the remote.
  + // If we have a system with sysid 0, it is the "fake" system we built to initialize the
  + // connection to the remote. We can remove it now that a remote system is discovered.
  if (_systems.size() == 1 && _systems[0].first == 0) {
  - LogDebug() << "New: System ID: " << static_cast<int>(message.sysid)
  - << " Comp ID: " << static_cast<int>(message.compid);
  - _systems[0].first = message.sysid;
  - _systems[0].second->system_impl()->set_system_id(message.sysid);
  + _systems.clear();
  }
  bool found_system = false;
  