nua_session.c: call soa_activate() only when nh->nh_soa is valid
nua_session.c: call soa_activate() only when nh->nh_soa is valid

darcs-hash:20081121141228-db55f-d7f4a351c482f6f9b8c9482a1eb776cf7a7468d6.gz

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