Add error check for GetCurrentSlot am: 1815c4dd52

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

Change-Id: I601e159bad36bac5459b9de1304a0b301f8a3bf6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>