Advance the cursor in data buffer 'length' bytes.

def advance(self, length):
        """Advance the cursor in data buffer 'length' bytes."""
        new_position = self._position + length
        if new_position < 0 or new_position > len(self._data):
            raise Exception('Invalid advance amount (%s) for cursor.  '
                            'Position=%s' % (length, new_position))
        self._position = new_position