fix bug in nat helper module
fix bug in nat helper module

BelledonneCommunications/flexisip,gale320/flexisip,BelledonneCommunications/flexisip,gale320/flexisip,biddyweb/flexisip,biddyweb/flexisip,gale320/flexisip,biddyweb/flexisip,BelledonneCommunications/flexisip,BelledonneCommunications/flexisip,biddyweb/flexisip,gale320/flexisip