Fix potential shifting past the end of the packet.
Fix potential shifting past the end of the packet.

We always assumed we could shift 2 into the "body" since:
option:0 = code
option:1 = len

Problem is we do this even when we got a code 255 or a padding, in this case,
there might not be anything to shift, and we would crash. In practice this would
_never_ happen since we use gigantic bufs and discard the rest, but it's still
wrong.

haesbaert/hdhcp,haesbaert/charrua-core