tport_type_udp.c: gracefully handle 0-size datagrams.
tport_type_udp.c: gracefully handle 0-size datagrams.

Problem reported by Michael Jerris.

darcs-hash:20070326170613-55b16-f79bbdd0902b149286b1c51964b651673c21b07a.gz

jart/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,jart/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,BelledonneCommunications/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,jart/sofia-sip