Fixed cube opengl example

Proper variable initialization should fix crashs and black screens for
that example.

Task-number: QTBUG-27281
Task-number: QTBUG-27021
Change-Id: I538f233ea3c2faf6c1864c46be0d03828e92a110
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

examples/opengl/cube/mainwidget.cpp

@@ -55,7 +55,8 @@ MainWidget::MainWidget(QWidget *parent) :
     QGLWidget(parent),
     timer(new QBasicTimer),
     program(new QGLShaderProgram),
-    geometries(new GeometryEngine)
+    geometries(new GeometryEngine),
+    angularSpeed(0)
 {
 }