Fix a crash with QGraphicsScene.

It happened when the scene gets deleted after ~QApplication has been
called.

test case:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QGraphicsScene *scene = new QGraphicsScene(&a);
    return 0;
}

Change-Id: I74d4023c9575242a2e334b2c786e00d0686c452b
Reviewed-on: http://codereview.qt.nokia.com/1544
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dietrich-de@nokia.com>

src/gui/graphicsview/qgraphicsscene.cpp

@@ -1639,7 +1639,8 @@ QGraphicsScene::~QGraphicsScene()
     Q_D(QGraphicsScene);
 
     // Remove this scene from qApp's global scene list.
-    qApp->d_func()->scene_list.removeAll(this);
+    if (!QApplicationPrivate::is_app_closing)
+        qApp->d_func()->scene_list.removeAll(this);
 
     clear();