[rlp] BasicEncoder produces incorrect results for iterators that are incorrect with their size hints

[skylerqpq]

An example of such an iterator:

struct MaliciousIterator<'a> {
    bytes: &'a [u8],
}

impl<'a> Iterator for MaliciousIterator<'a> {
    type Item = u8;
    fn size_hint(&self) -> (usize, Option<usize>) {
        (20, Some(20))
    }

    fn next(&mut self) -> Option<Self::Item> {
        match self.bytes.split_first() {
            Some((first, rest)) => {
                self.bytes = rest;
                Some(*first)
            }
            None => None,
        }
    }
}