vec_deque::Iter has unsound Debug implementation

[RalfJung]

Found by @MaloJaffre: The following code causes UB (not observable through crashes, but still):

use std::collections::VecDeque;
fn main() {
    println!("{:?}", VecDeque::<u32>::new().iter());
}

This will create a VecDeque ring with capacity 8, then turn that into a slice for Iter, and then print that entire slice. Run it in Miri to see for yourself (Miri is in the "tools" menu).

[nluedtke]

This was assigned CVE-2019-1010299.