Fix wrong assert in QSGRenderLoop::setInstance

The assert before test not if the scoped pointer is null but the pointer
which was holding the scoping pointer.

Task-number: QTBUG-36932
Change-Id: I7a5e51752446556c1796bb677fe69cbceffdd3e0
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

src/quick/scenegraph/qsgrenderloop.cpp

@@ -204,7 +204,7 @@ QSGRenderLoop *QSGRenderLoop::instance()
 
 void QSGRenderLoop::setInstance(QSGRenderLoop *instance)
 {
-    Q_ASSERT(!s_renderLoopInstance);
+    Q_ASSERT(s_renderLoopInstance->isNull());
     s_renderLoopInstance->reset(instance);
 }