Return the start inside the m_attributes array for a given attribute

def _get_attribute_offset(self, index):
        """
        Return the start inside the m_attributes array for a given attribute
        """
        if self.m_event != START_TAG:
            log.warning("Current event is not START_TAG.")

        offset = index * ATTRIBUTE_LENGHT
        if offset >= len(self.m_attributes):
            log.warning("Invalid attribute index")

        return offset