QTcpSocket on Windows: calling disconnectFromHost inside slot connected to bytesWritten reports error

according to https://codereview.qt-project.org/#change,80719 (stabilizing tst_qnetworkreply):

If you call QTcpSocket::disconnectFromHost() from within bytesWritten() , the counterpart reports an error (Connection closed) on Windows.

Whether that is a bug or not I cannot judge (this is similar to QProcess::error() being emitted from within start() on Windows and these kinds of subtle issues).

The fix stabilizes the test and fixes a leak, I leave it to the network experts to judge / fix the issue.