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

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

Change-Id: I86c442ac327ad367df38008a2e5fa6eefa8f338d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>