slayers: SCION does not notice truncated payload

[shitz]

The SCION header does not check that the payload is not truncated when decoding.

E.g. IPv6 does the check like this:
https://github.com/google/gopacket/blob/914d8955fc258b9a501fa5e227117c4cd293f981/layers/ip6.go#L270-L275