Fix xor writer failure am: cb9932ff76 am: 5c606ab6b3

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

Change-Id: I8741ad7fd30219dc158f7a2f78384a7b9606565f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>