Add error check for GetCurrentSlot am: 1815c4dd52 am: 80bdc44943

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

Change-Id: If1d745ed46bbb19eb7ad0d47debc4ed2aefbac9f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>