Fix OgreBillboardChain element counting (#607)
Fix OgreBillboardChain element counting (#607)

- getNumChainElements now returns 0 if chain is empty
- getChainElement throws an exception when chain is empty exhibiting the same behaviour as updateChainElement
OGRECave/ogre,paroj/ogre,OGRECave/ogre,OGRECave/ogre,paroj/ogre,paroj/ogre,paroj/ogre,OGRECave/ogre,paroj/ogre,OGRECave/ogre