nta.c: avoid extra work when a response is internally generated.
nta.c: avoid extra work when a response is internally generated.

darcs-hash:20070326180126-55b16-858b781a8cd0c055a04e8fc948a313f69639d839.gz

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