title: QDataStream not increasing position of QByteArray
body: <p>When I write raw network bytes to a QDataStream object repeatedly, the QDataStream never increases the write position of the underlying QByteArray.</p>
  
  <p>Here's the code:</p>
  
  <pre><code>QByteArray recvBuf;
  int bytePosition = 0;
  
  void init( ){
      recvBuf = QByteArray( 240 * 10, Qt::Uninitiliazed );
  }
  
  void receiveAppend( ){    
      QByteArray buffer( 240, Qt::Uninitialized );
      QDataStream datastream( recvBuf, QIODevice::WriteOnly );
      datastream.setByteOrder( QDataStream::LittleEndian );
  
      if( udpSocket-&gt;readDataGram( buffer.data(), buffer.size(), &amp;ip, &amp;port ){
          datastream.device().seek( bytePosition );
          datastream.writeRawData( buffer.data(), buffer.size() );
          bytePosition += 240;
      }
  }
  </code></pre>
  
  <p>Why is this happening?</p>
  
  <p>The code works fine on Linux Mint 18 64Bit. But it's not working on Windows 10 64Bit.<br>
  I'm using Qt Creator 4.3.1 with Qt 5.9.1.</p>