If k==0, log==0 can indicate that the coded value uses more than
MIN_CACHE_BITS bits.  With MIN_CACHE_BITS==32, the fast branch is
incorrectly taken in this case unless explicitly forbidden.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@17550 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b