QObjCWeakPointer: Clear existing associated object on assignment

If the existing m_object is the same as the incoming object the
call to objc_setAssociatedObject in trackObjectLifetime will
release the existing WeakPointerLifetimeTracker after assigning
a new one, which means we'd clear the QObjCWeakPointer's object.

We now reset the state up front.

Fixes: QTBUG-132256
Pick-to: 6.8 6.9
Change-Id: If2c08840d465ae6d190c87a4720a537fe9caa8dc
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/corelib/kernel/qcore_mac_p.h

@@ -504,6 +504,9 @@ public:
     QObjCWeakPointer &operator=(const QObjCWeakPointer &other)
     {
         QMacAutoReleasePool pool;
+#if !USE_OBJC_WEAK
+        objc_setAssociatedObject(m_object, this, nil, OBJC_ASSOCIATION_RETAIN);
+#endif
         m_object = other.m_object;
 #if !USE_OBJC_WEAK
         trackObjectLifetime();