optimize init_vlc().

optimize init_vlc().
Reduce worst case time from O(N^2) to O(N*log(N)).
Speedup average case by a factor of 10 in ffv2 (total decoding speed +4-25%),
factor of 1.3 in ffvhuff (total +0.5%),
factor of 1.8 in indeo5 (total +1%),
factor of 1.1 in mjpeg (total +0.1%).

Originally committed as revision 22715 to svn://svn.ffmpeg.org/ffmpeg/trunk