zeroize: fixup rust 1.92 warnings

[baloo]

#[derive(ZeroizeOnDrop)] was throwing warnings like:

warning: value assigned to `curve` is never read
  --> src/crypto/ecdsa.rs:43:9
   |
43 |         curve: ECCCurve,
   |         ^^^^^
   |
   = help: maybe it is overwritten before being read?
   = note: `#[warn(unused_assignments)]` (part of `#[warn(unused)]`) on by de

This changes the tests to highlight the new warnings.

[baloo]

If possible, a zeroize_derive 1.5.1 release with that PR would be appreciated!

[tarcieri]

@baloo hmm, 1.5.0 is still unreleased, and includes an MSRV bump

We should probably get new releases of zeroize and zeroize_derive with that bump out, though I expect some pushback.

[baloo]

Ha, right! I didn't even check.

Maybe I should backport this to 1.4 then.

[baloo]

I don't really have a way to open a PR for a backport because there is no branch to target to.

I've made a commit:

• https://github.com/RustCrypto/utils/compare/zeroize_derive-v1.4.2...baloo:push-mtmtvkvwzpzz?expand=1
• baloo@ba99c6d

If you could push a 1.4.3, that would be appreciated!

I've checked that commit on rpgp (and 1.92), and it doesn't report any warnings anymore.