Don't delete scene-graph atlas manager if it hasn't been created yet
Don't delete scene-graph atlas manager if it hasn't been created yet

QSGContext::invalidate() is called from the QSGContext destructor,
but the atlas manager is only created if QSGContext::initialize()
is called, which may not always happen.

The result was a bunch of "QCoreApplication::postEvent: Unexpected
null receiver" messages during testing.

Change-Id: I11a294d48c06d687a9f82cf1ce825063cda8ecf6
Reviewed-by: Gunnar Sletta <d0b71651f3efaa9fda200bbcc395b7b82a871fdd@digia.com>

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