No-thread: prevent double delete of QThreadPrivate

Change-Id: Ie34c06ad798be6bd91f5c356051daaa72800c20a
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthread.cpp

@@ -879,6 +879,7 @@ QThreadPrivate::QThreadPrivate(QThreadData *d) : data(d ? d : new QThreadData)
 
 QThreadPrivate::~QThreadPrivate()
 {
+    data->thread = nullptr; // prevent QThreadData from deleting the QThreadPrivate (again).
     delete data;
 }