UB in Collect:: reserve_get_tail_slice

It appears that [Collect::reserve_get_tail_slice](https://github.com/rayon-rs/rayon/blob/master/src/iter/collect/mod.rs#L139) might have undefined behavior. The code is reserving uninitialized space, then creating a mutable reference to it. This us UB according to this [documentation](https://doc.rust-lang.org/stable/reference/behavior-considered-undefined.html).