nta.c: discard stateless 2XX response to INVITE with bad Via header
nta.c: discard stateless 2XX response to INVITE with bad Via header

The responses with zero or more than one Via headers or Via header not
matching agent's list of Via headers are now discarded.

Problem reported by Fabio Margarido.

darcs-hash:20070305150655-55b16-14c2145d2bbb3d003e1dac2069f356e06e7fb27e.gz

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