Fix assertion

[YichiZhang0613]

In this assertion, according to the semantics, self.total_len should not be 0. The message reflects it as it requires "Total length must be positive".

fn next(&mut self) -> Option<Self::Item> {
        assert!(
            self.total_len >= 0.0,
            "Total length must be positive. Total_len: {}",
            self.total_len
        );