Use ZeroMemory on Windows instead of zero memset

See the documentation for details:

http://msdn.microsoft.com/en-us/library/windows/desktop/aa366920(v=vs.85).aspx

Two immediate advantages:

1) Consistent with the rest of Qt.

2) It is more convenient and explicit than the raw memset.

Unfortunately, Windows CE does not seem to have this supported, so it is not
changed in there:

http://users.atw.hu/lingoland/akademia/zeromem.htm

"Windows CE: Unsupported."

Change-Id: I44154c320f13054e1bfd08e3ff1e957430eeeb49
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>