sip-date.c: silence Coverity warning
sip-date.c: silence Coverity warning

MeeGo Coverity issue CID#7566

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