Crash in QWidgetPrivate::unregisterOleDnd() when dropTarget->Release() decrements reference count to zero

This is a duplicate of QTBUG-7801, but that bug was closed due to lack of activity.

A crash will occur in QWidgetPrivate::unregisterOleDnd (\src\gui\kernel\qwidget_win.cpp) when the function attempts to call CoLockObjectExternal(dropTarget, false, true) if the dropTarget object's reference count was decremented to zero by the dropTarget->Release() call at the beginning of the function.

I haven't been able to create a small project that can repro this issue, but I have attached a patch which fixes the problem for our project.