nua_session.c: fixed possible segfault in signal_call_state_change()
nua_session.c: fixed possible segfault in signal_call_state_change()

Thanks for Michael Jerris for reporting the problem.

darcs-hash:20061208080532-65a35-e67f97abb169bd359a1df44f4caa56b639114083.gz

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