Clear unused members of the overlapped structure before each usage

For more detail, see remarks from the MSDN:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms684342%28v=vs.85%29.aspx

Change-Id: Id6e5adc48e8a4648f1aabba598696b34c658a159
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>