harden offset decoding when deserializing

[ralexstokes]

https://github.com/ralexstokes/ssz-rs/pull/57/files#r1218449340

h/t @doubledup

I am a bit surprised there is no spec test for this but let's go ahead and verify ourselves

[ralexstokes]

refer #60

[ralexstokes]

places to verify:

• deserializing "variable" types
• including the derived code

[ralexstokes]

I think between #60 and #57, this issue is addressed