VecDeque benchmarks have UB

[RalfJung]

This benchmark function has UB:

rust/library/alloc/src/collections/vec_deque/tests.rs

Lines 29 to 40 in bc3fae4

	}
	#[bench]
	#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
	fn bench_pop_back_100(b: &mut test::Bencher) {
	let mut deq = VecDeque::<i32>::with_capacity(101);
	b.iter(|| {
	deq.head = 100;
	deq.tail = 0;
	while !deq.is_empty() {
	test::black_box(deq.pop_back());

It directly accesses private fields of VecDeque to set the head and tail pointers, but doesn't initialize the element in the middle so pop_back reads uninitialized memory.