Fix partial read failure on last block of src partition am: a3a68a94ba am: 94edb2fcca am: a99c0cf6bd

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

Change-Id: I8beed1cdbbc130c732fad393ea71b44c532e3e07
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>