message: fix one more unaligned self.ptr, and add tests
diff: //@compile-flags: -Zmiri-strict-provenance
  +#![feature(iter_advance_by, iter_next_chunk)]
  +
  // Gather all references from a mutable iterator and make sure Miri notices if
  // using them is dangerous.
  fn test_all_refs<'a, T: 'a>(dummy: &mut T, iter: impl Iterator<Item = &'a mut T>) {
  @@ -44,6 +46,18 @@ fn vec_into_iter_zst() {
  for _ in vec![[0u64; 0]].into_iter() {}
  let v = vec![[0u64; 0], [0u64; 0]].into_iter().map(|x| x.len()).sum::<usize>();
  assert_eq!(v, 0);
  +
  + let mut it = vec![[0u64; 0], [0u64; 0]].into_iter();
  + it.advance_by(1);
  + drop(it);
  +
  + let mut it = vec![[0u64; 0], [0u64; 0]].into_iter();
  + it.next_chunk::<1>().unwrap();
  + drop(it);
  +
  + let mut it = vec![[0u64; 0], [0u64; 0]].into_iter();
  + it.next_chunk::<4>().unwrap_err();
  + drop(it);
  }
  fn vec_into_iter_rev_zst() {
  