nta.c: fix leak in nta_msg_ackbye()
nta.c: fix leak in nta_msg_ackbye()

MeeGo Coverity issue CID #11774

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