nimbro_topic_transport: udp_receiver: protect against unknown types
nimbro_topic_transport: udp_receiver: protect against unknown types

previously, we would die attempting to convert the md5sum...

AIS-Bonn/nimbro_network,AIS-Bonn/nimbro_network,dreuter/nimbro_network,AIS-Bonn/nimbro_network,dreuter/nimbro_network,dreuter/nimbro_network