But the PDU length checks back, so "tmp" doesn't go negative.

But the PDU length checks back, so "tmp" doesn't go negative.

At least as I read RFC 3847, you won't get a system ID without the
remaining hold time field; after printing the flags field, check whether
there's anything else after the flag field and, if not, quit, otherwise
print the remaining hold time field and, if present, the system ID field.