s2sip.c: check for NULL argument in s2_sip_update_dialog()
s2sip.c: check for NULL argument in s2_sip_update_dialog()

MeeGo Coverity issue CID #11757

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