message: refactor: use channel index to reference port shutdown
  Make use of the channel index to reference the shutdown port index to make it consistent
  with the initialized check.
  This is because even channel indexes are for data and odd indexes are for errors.
diff: @@ -310,15 +310,13 @@ where
  .map_err(Error::InvalidErrorMessage)?;
  sender.send(s).map_err(Error::ForwardErrorMessage)?;
  }
  - } else {
  - if !chan_state[port_index].shutdown {
  + } else if !chan_state[ch].shutdown {
  writers[port_index]
  .write_all(&bytes)
  .await
  .map_err(Error::WriteBytesFromPod)?;
  }
  }
  - }
  Message::ToPod(ch, bytes) => {
  let mut bin = Vec::with_capacity(bytes.len() + 1);
  @@ -335,9 +333,9 @@ where
  return Err(Error::InvalidChannel(ch));
  }
  let port_index = ch / 2;
  - if !chan_state[port_index].shutdown {
  + if !chan_state[ch].shutdown {
  writers[port_index].shutdown().await.map_err(Error::Shutdown)?;
  - chan_state[port_index].shutdown = true;
  + chan_state[ch].shutdown = true;
  closed_ports += 1;
  }
  