don't throw ogre exception when receiving invalid table

Although it should not happen, the plugin should still verify it's input
messages before passing them on to OGRE. Otherwise rviz crashes:

rviz: /build/buildd/ogre-1.7.4/OgreMain/src/OgreNode.cpp:413: virtual void 
Ogre::Node::setPosition(const Ogre::Vector3&): Assertion `!pos.isNaN() &&
"Invalid vector supplied as parameter"' failed. Aborted (core dumped)