QWin32PrintEnginePrivate check for NULL pDevMode and hdc.

The MSDN documentation states that the pDevMode member of
PPRINTER_INFO_2 may be NULL. Also, CreateDC may fail and return
a NULL. Rework release() to release resources even if hdc was null.

Task-number: QTBUG-39373
Change-Id: Ia08da61bf6ab99f02f0c7a09c608a5d3db34ef65
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Fix compilation on Windows without precompiled headers

qnetworkinterface_win_p.h needs to include at least one Qt header before
it can use QT_BEGIN_NAMESPACE. That header is the first header in
qnetworkinterface_win.cpp.

Found when trying to compile Qt with ICC.

Change-Id: Iaa312ff54243b6fb3beb107f0eda74f92c6e3ebb
Reviewed-by: Richard J. Moore <rich@kde.org>