Check that the payload is long enough (at least 2 bytes).

def pre_dissect(self, s):
        """
        Check that the payload is long enough (at least 2 bytes).
        """
        length = len(s)
        if length < _BGP_CAPABILITY_MIN_SIZE:
            err = " ({}".format(length) + " is < _BGP_CAPABILITY_MIN_SIZE "
            err += "({})).".format(_BGP_CAPABILITY_MIN_SIZE)
            raise _BGPInvalidDataException(err)
        return s