Compute the 'sources_number' field when needed

def post_build(self, packet, payload):
        """Compute the 'sources_number' field when needed"""
        if self.sources_number is None:
            srcnum = struct.pack("!H", len(self.sources))
            packet = packet[:26] + srcnum + packet[28:]
        return _ICMPv6.post_build(self, packet, payload)