Remove unnecessary `add` in `clone_from_impl`

[JustForFun88]

The guard.0 = index + 1 in the implementation of clone_from _impl is completely unnecessary, since an invalid index can be set in advance.

[Amanieu]

The + 1 is implicitly calculated by the iterator anyways, so it doesn't cost anything extra in optimized builds.

I prefer the original form of the code since guard.0 clearly indicates the point that separates the initialized part of the array vs the uninitialized part. This also avoids the need to special-case usize::MAX.