Replace the zero handling in extend_to_full_distribution.

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://code.google.com/p/webm/issues/detail?id=1089

Change-Id: Ia6dcc8922e1acbac0eeca23a4d564a355c489572