osgviewerQt crashed when press key F

[x1244]

When running osgviewerQt, press the key F, the application crashed. The reason was osgOpenGLWidget::keyPressEvent don't check the validation of _mainwindow when invoke _mainwindow->setCentralWidget(this). I have fixed it and will make a pull requests.

            showNormal();
            setMinimumSize(g);
            QMainWindow* _mainwindow = dynamic_cast<QMainWindow*>(parent());
            if(_mainwindow){
                _mainwindow->setCentralWidget(this);
            }

            if(layout())
                layout()->setContentsMargins(sMargins);

            qApp->processEvents();
            setMinimumSize(QSize(1, 1));

[mathieu]

merged PR