zeroize 1.9.0: the optimization_barrier non-asm fallback reads possibly uninitialized memory as u8 (UB from safe code) #1549

On targets without stable asm! (for example wasm32-unknown-unknown), and under
Miri, optimization_barrier<T: ?Sized>(val: &T) (src/barrier.rs:92-100) calls
custom_black_box(ptr.cast::<u8>()), which does core::ptr::read_volatile(p) with
p: *const u8. If byte 0 of *val is uninitialized, producing that u8 is
Undefined Behaviour. Byte 0 is uninitialized for padding, for MaybeUninit, or for the
payload bytes of a niche-encoded enum after a typed write.

optimization_barrier is a safe pub fn, so safe code can trigger this, e.g.
zeroize::optimization_barrier(&core::mem::MaybeUninit::<u8>::uninit()).

We see that #1535 removed the internal callers, but barrier.rs on master is unchanged.

Suggested fix: read_volatile(p.cast::<core::mem::MaybeUninit<u8>>()), or drop
custom_black_box and rely on core::hint::black_box alone.

Could this be included in 1.9.1?