Upgrade libprotobuf-mutator to 8942a9ba43d8bb196230c321d46d6a137957a719 am: 81735e2127

Original change: https://android-review.googlesource.com/c/platform/external/libprotobuf-mutator/+/1415390

Change-Id: Ib968917a595c42655106a1c6fcb5736ee04abd76