SceneNode::removeAndDestroyAllChildren: fix usage of invalid end() iter
SceneNode::removeAndDestroyAllChildren: fix usage of invalid end() iter

resolves https://ogre3d.atlassian.net/projects/OGRE/issues/OGRE-577 
paroj/ogre,paroj/ogre,paroj/ogre,paroj/ogre,paroj/ogre,OGRECave/ogre,OGRECave/ogre,OGRECave/ogre,OGRECave/ogre,OGRECave/ogre