QSGRenderLoop: don't dereference gl pointer if it has been deleted
QSGRenderLoop: don't dereference gl pointer if it has been deleted

Change-Id: Icd612b243cdfe1248d1b94964c53f5102f9558d2
Reviewed-by: Gunnar Sletta <d0b71651f3efaa9fda200bbcc395b7b82a871fdd@digia.com>

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