Unsafe access by index may panic

[kpcyrd]

I've fuzzed sniffglue with cargo-fuzz for a couple of days and it discovered a panic in the packet processing. The panic occures inside dhcp4r, which is used for dhcp decoding. The problem seems to be an out of range access in dhcp4r::packet::decode.

The packet that triggers the bug can be found here: kpcyrd/sniffglue#16

You may want to look into nom, as it's easier to decode packets and you get length checks for free. :)

[krolaw]

I suggest adding check in the packet function ensuring the message length is at least 240 bytes.

[lkolbly]

This should fixed in 0.2.0 (we use nom now), I've made a PR to update sniffglue: kpcyrd/sniffglue#61