Fix memory leak with value types

Commit 3b14e2f replaced the
QQmlRefPointer<QQmlPropertyCache> with a raw QQmlPropertyCache pointer
and added a V4_NEEDS_DESTROY tag. However unfortunately the destroy()
method in the heap class does not decrease the reference count.

Change-Id: I90a8c56cd638592b67aae7041fbb57c879c4146c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/qml/qml/qqmlvaluetypewrapper.cpp

@@ -96,6 +96,8 @@ void Heap::QQmlValueTypeWrapper::destroy()
         valueType->metaType.destruct(gadgetPtr);
         ::operator delete(gadgetPtr);
     }
+    if (_propertyCache)
+        _propertyCache->release();
     Object::destroy();
 }