[automerger] Check NAL size before looking inside am: e0c020969d am: 8c4ae8057c am: 767a82a9d6 am: f7d9bba9b1 am: e04b7c8c06 am: 25e199f787 am: 93962ef505

Change-Id: Ibdf718fcf9a6494c3edaf73952c9723e155e98fd