Panic when decoding an invalid UTCTime

[nmathewson]

Hello, I hope this is the right place to report this; I didn't find any documentation for a preferred method for reporting security issues.

The following code panics when trying to parse an invalid UTCTime object:

fn main() {
    let input =  [55, 13, 13, 133, 13, 13, 50, 13, 13, 133, 13, 13, 50, 13, 133];
    let output = simple_asn1::from_der(&input);
    println!("{:?}", output);
}

The panic occurs because of these line in lib.rs:

                let v = String::from_iter(body.iter().map(|x| *x as char));
                let y = &v[0..2];

If the string is constructed in such a way that the first two bytes do not end on a character boundary, the slice operation will panic.

Found by fuzzing a downstream library.

I'll submit a patch ASAP.