Buffer overflow in crossbeam skiplist might need advisory

[Shnatsel]

crossbeam-rs/crossbeam#940

[taiki-e]

It still reads the valid values that we allocated, so I don't think it causes security issues like buffer overflow right away. However, the informational="unsound" advisory may make sense since it definitely violated the get_unchecked's safety contract.

(In any case, I should release the fix soon...)

[Shnatsel]

Thanks for the context! In that case I don't think an advisory is necessary. This may still pose some issues wrt aliasing, but there's a separate issue tracking aliasing issues: crossbeam-rs/crossbeam#878

[Shnatsel]

Besides that, I'm not thrilled about this being an Index implementation instead of a separate unsafe fn. While this probably doesn't cause any issues for end users now, this makes the code easy to mess up when making any changes in the future, so I wouldn't be surprised if I find myself filing an advisory about crossbeam later.