message: put sendstream state tx_buf_mut after tx_buf
diff: @@ -136,7 +136,7 @@ enum SendStreamState {
  }
  impl SendStreamState {
  - fn tx_buf_mut(&mut self) -> Option<&mut TxBuffer> {
  + fn tx_buf(&self) -> Option<&TxBuffer> {
  match self {
  SendStreamState::Send(buf) => Some(buf),
  SendStreamState::DataSent(buf, _) => Some(buf),
  @@ -147,7 +147,7 @@ impl SendStreamState {
  }
  }
  - fn tx_buf(&self) -> Option<&TxBuffer> {
  + fn tx_buf_mut(&mut self) -> Option<&mut TxBuffer> {
  match self {
  SendStreamState::Send(buf) => Some(buf),
  SendStreamState::DataSent(buf, _) => Some(buf),
  