[NFC][scudo] Fix sign-compare warning in test am: c1d01dffbd am: 1b0c5f9236 am: d8386172de

Original change: https://android-review.googlesource.com/c/platform/external/scudo/+/1729760

Change-Id: I3b81aa860e4864e2540f93243a546716f9018440