[automerger skipped] Fix a potential wild pointer error when markSlotSuccessful comes late am: 28d49bc315 -s ours am: c8b92aab58 -s ours

am skip reason: skip tag Change-Id I12c01c5f062ec4b41718f2cada817f6557eb2b0e with SHA-1 65e8b6c195 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/update_engine/+/14078192

Change-Id: I690d8b72a89eaf830a4b95b0bc7123d888b9371b