lz4diff: Check array bounds prior to access am: 4203702caa am: 06c67e0935 am: 7bbe7b4e8b

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1936320

Change-Id: I462a1f8abd311bb819837a674d3f8f4b41968154