Ensure the given start/end fragment indices make sense:
        if one of them is ``None`` (i.e., not specified),
        then set it to ``0`` or ``len(self)``.

def _check_min_max_indices(self, min_index=None, max_index=None):
        """
        Ensure the given start/end fragment indices make sense:
        if one of them is ``None`` (i.e., not specified),
        then set it to ``0`` or ``len(self)``.
        """
        min_index = min_index or 0
        max_index = max_index or len(self)
        if min_index < 0:
            self.log_exc(u"min_index is negative", None, True, ValueError)
        if max_index > len(self):
            self.log_exc(u"max_index is bigger than the number of intervals in the list", None, True, ValueError)
        return min_index, max_index