Fix QtSharedPointer::ExternalRefCountData object leak in v4 engine

Always call ~QObjectWrapper() to the wrapper object in
QObjectWrapper::destroy(), otherwise the m_object member of QObjectWrapper
may still hold a QtSharedPointer::ExternalRefCountData object which will
never been deleted.
I don't know why this will not cause leak in the past, but it seems that
the leak was introduced in 7ae796cb141b73a1b215b2b0fd64b7ffbbd1e510

Change-Id: I24b49bb11f95b7e3060c7adba1ab80b615da2942
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>