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)
Merge pull request #11 from v4hn/table-nan-no-ogre-exception

don't throw ogre exception when receiving invalid table