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>