from_elem is unsound when clone can panic

[mbrubeck]

If clone panics during SmallVec::from_elem then uninitialized memory is dropped. [Original report by dbaupp on Reddit.]

This bug was introduced by #93 which is not yet included in the latest published release (0.6.2).

The suggested solution is to use something like SetLenOnDrop to make sure the length is set correctly when destructors run, without inhibiting optimizations.

[Phosphorus15]

Dropping unintialized memory could be considered a vulnerability, which is equivalent to use after free. I suggest submitting a ticket to Rust security advisory database just like #96 do. @Shnatsel @mbrubeck

[Shnatsel]

This bug never made it into a release. No dependencies were ever affected, so no need for an advisory.

We still have #156 outstanding though