nua_session.c: fix crash if session was terminated while INVITE was in progress
nua_session.c: fix crash if session was terminated while INVITE was in progress

darcs-hash:20080516142826-db55f-8df439d30dd8ffe2fe7c01dc487d7b2276d49aaf.gz

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