nta.c: check for su_zalloc() result
nta.c: check for su_zalloc() result

MeeGo Coverity issue CID#7660

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