SmallVec < 0.6.13 is unsound

[RalfJung]

servo/rust-smallvec#162 fixes a soundness bug in SmallVec where mem::uninitialized was used incorrectly. This fix is included in 0.6.13, but previous versions are unsound. The latest version is 1.4, but given how widely used this crate is, it might still be worth having an advisory for the old broken versions.

The advisory should probably follow whatever policy ends up being in place for soundness bugs.