Hello GL2 example: Fix exit crash

Bail out of cleanup() when run 2nd time.

Task-number: QTBUG-60626
Change-Id: I8a9be2fcfb0e8a5584ce8ed7952affff24bd2a33
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

examples/opengl/hellogl2/glwidget.cpp

@@ -128,6 +128,8 @@ void GLWidget::setZRotation(int angle)
 
 void GLWidget::cleanup()
 {
+    if (m_program == nullptr)
+        return;
     makeCurrent();
     m_logoVbo.destroy();
     delete m_program;