Fix xor writer failure am: cb9932ff76

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2397475

Change-Id: Ia4f388155f17bed48f1f750540246b07af4a5c9f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>