Make the zero handling in extend_to_full_distribution more explicit.
The old workaround "p = 0 ? 0 : p -1" is misleading. ?: happens before = assigning back to p truncates to one byte. Therefore it is equivalent to (p - 1) & 0xFF, but the check just exists to work around a first pass bug, so let's make the work around more clear. https://bugs.chromium.org/p/webm/issues/detail?id=1089 Change-Id: I587c44dd61c1f3767543c0126376f881889935af