Check that the payload is long enough to build a NTP packet.

def pre_dissect(self, s):
        """
        Check that the payload is long enough to build a NTP packet.
        """
        length = len(s)
        if length < _NTP_PACKET_MIN_SIZE:
            err = " ({}".format(length) + " is < _NTP_PACKET_MIN_SIZE "
            err += "({})).".format(_NTP_PACKET_MIN_SIZE)
            raise _NTPInvalidDataException(err)
        return s