Add error check for GetCurrentSlot am: 1815c4dd52

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

Change-Id: If038a57df61a15ab6bf6672b2ea504792ec03745
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>