Add error check for GetCurrentSlot am: 1815c4dd52 am: 642dbc7b33

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2820039

Change-Id: I688b72432c52c9ac6f1e73def7d58e561b654303
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>