Don't try to decode options when the packet is completely empty (reported by @kit-ty-kate)

[dinosaure]

@kit-ty-kate reported a crash which comes from utcp. This is the trace:

console 2026-01-26T13:34:22-00:00: Fatal error: exception Invalid_argument("Cstruct.sub: [34,20](1514) off=20 len=-20")
console 2026-01-26T13:34:22-00:00: Raised at Stdlib.invalid_arg in file "[stdlib.ml](http://stdlib.ml/)", line 30, characters 20-45
console 2026-01-26T13:34:22-00:00: Called from Utcp__Segment.decode in file "duniverse/utcp/src/segment.ml", line 664, characters 20-73

From what I understand, we received an completely empty packet (no data, no options). This PR add a just a guard to ensure that we can Cstruct.sub. Otherwise, we returns an empty list of options.