Compute the 'records_number' field when needed

def post_build(self, packet, payload):
        """Compute the 'records_number' field when needed"""
        if self.records_number is None:
            recnum = struct.pack("!H", len(self.records))
            packet = packet[:6] + recnum + packet[8:]
        return _ICMPv6.post_build(self, packet, payload)