kani: Investigate failed check in memcmp

This was discovered in #1332, but also occurs in a synchronous context:

#[kani::proof]
pub fn main() {
    let coll: Result<Vec<u32>, ()> = std::iter::empty().collect();
    assert_eq!(Ok(vec![]), coll);
}

Running this with Kani gives the result:

SUMMARY:
 ** 2 of 436 failed
Failed Checks: memcmp region 1 readable
 File: "<builtin-library-memcmp>", line 19, in memcmp
Failed Checks: memcpy region 2 readable
 File: "<builtin-library-memcmp>", line 21, in memcmp

VERIFICATION:- FAILED