[NFC][scudo] Fix sign-compare warning in test am: c1d01dffbd am: 1b0c5f9236 am: d8386172de am: 18c7ed1d7f

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

Change-Id: I7c9018995d088a0ad2b30216c6d80191d287f954