Fix a few warnings (signed/unsigned comparison in gtest, and missing field initializers) am: 6841f6e058 am: 6b63e40153

Original change: https://android-review.googlesource.com/c/platform/external/gwp_asan/+/1860473

Change-Id: Ib442016af445cc7ff6b259d1d4944116b05fd689