Also fixed an arithmetic issue where a 0 0 3 at the end of the rbsp would include the 3 (that's not a legal bitstream anyway, so it probably wasn't a real bug, but it was incorrect).
This maintains the underflow fix from an earlier CL (https://codereview.webrtc.org/1219493004/). The overflow fix is virtually impossible to hit (hence no unit tests), but is there for strict correctness.
BUG=
Review URL: https://codereview.webrtc.org/1226203002
Cr-Commit-Position: refs/heads/master@{#9581}
43e7d3bc15