message: IPv4: use saturating_sub() to avoid "attempt to subtract with overflow"
diff: @@ -220,7 +220,7 @@ fn ipv4_options_length(ipv4: &Ipv4Packet) -> usize {
  // the header_length unit is the "word"
  // - and a word is made of 4 bytes,
  // - and the header length (without the options) is 5 words long
  - ipv4.get_header_length() as usize * 4 - 20
  + (ipv4.get_header_length() as usize * 4).saturating_sub(20)
  }
  #[test]
  