- validate TCP header langth against IP payload and see if it actually
  fits or not, return on error

of uint16_t pointer casts)

tcp_process_ws

…lta.

…mbers, preventing unsigned wraparound in bytes_in_flight

…cksum update

- add tcp_seq_inc() helper for wrap-safe sequence number arithmetic,
  replacing bare ee32(tcp->seq) + 1 in all five call sites
- fold carry in ICMP echo reply checksum update to avoid uint16_t truncation

fits in the ip payload