Potential multiplication overflow when calculating length of NdpOptionPacket

[heikki-heikkila]

In https://github.com/libpnet/libpnet/blob/main/pnet_packet/src/icmpv6.rs#L304, length calculation is as follows:

    /// Calculate a length of a `NdpOption`'s payload.
    fn ndp_option_payload_length(option: &NdpOptionPacket) -> usize {
        let len = option.get_length();
        if len > 0 {
            ((len * 8) - 2) as usize
        } else {
            0
        }
    }

Value "len" is of type u8, so multiplication "len * 8" is done with 8-bit unsigned arithmetic. If len is 32 or more, the result is overflow; this is UB. The correct calculation, of course, is "len as usize * 8".