Fix access to invalid memory in ~QQmlEnginePrivate
Fix access to invalid memory in ~QQmlEnginePrivate

~QQmlEnginePrivate tries to set incubationController->d to 0. However,
we're already setting the backpointer (q) of incubationController to 0
in ~QQmlEngine, so that the IncubationController destructor might run
in between without being able to reset incubationController.

To fix this, just handle the unsetting of incubationController once, in
~QQmlEnginePrivate.

Task-number: QTBUG-32161
Change-Id: I43a72dcfc0f95938ebfa67d3998adff1fb2d05a1
Reviewed-by: Alan Alpert <da25f2afbbb946022229108d81aeba0c6b35c5c3@blackberry.com>

mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative