Fix partial read failure on last block of src partition am: a3a68a94ba am: 94edb2fcca

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2525819

Change-Id: Iedf9d2d3a7aa0d3c8be7dd02ef7149fefc3fca95
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>