Fix PIN length check

[robin-nitrokey]

@sosthene-nitrokey in #62 (comment):

Minor issue of spec compliance:

Minimum PIN Length: 4 code points.

In decrypt_pin_check_length we check 4 bytes, not 4 code points. The max is still 63 bytes, so the check should be:

        let pin_length = pin.iter().position(|&b| b == b'\0').unwrap_or(pin.len());
        let pin_data = &pin[..pin_length];
        let pin_utf8 = core::str::from_utf8(pin_data)?;
        let pin_len_codepoints = pin_utf8.chars().count();

        if pin_len_codepoints < 4 || pin_length > 63{
            return Err(Error::PinPolicyViolation);
        }