[NFC][scudo] Fix sign-compare warning in test am: 5a56cb4682 am: 2708b1b341 am: 906751ed5a am: 00c960b755

Original change: https://android-review.googlesource.com/c/platform/external/scudo/+/1729756

Change-Id: Ie7f35f8d1c1e477519a8cc2d43b6712f7fa5f088