fixing illegal 3. esc bug (the mpeg4 std only requires encoders to use...

fixing illegal 3. esc bug (the mpeg4 std only requires encoders to use unescaped symbols but not esc1 or esc2 if they are shorter than esc3, andjust beause its logical to use the shortest possible vlc doesnt mean encoders do that)

Originally committed as revision 1304 to svn://svn.ffmpeg.org/ffmpeg/trunk