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

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

Change-Id: I27442b2db02596bf02363facc1f3d03326973dbe