The "bug" is from assuming you are parsing a valid packet. The option length field is the length of the option legnth and option data combined. The minimum value it can have is 1 which is when the length of the option data is 0.

cp[IPOPT_OLEN] refers to the option length. Your delta variable is the length of the option data. The confusing part is that in most network protocols length fields do not count the size of the length number.