Make the zero handling in extend_to_full_distribution more explicit.

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