Use ff_msmpeg4_decode_init to initialize ff_msmp4_mb_i_vlc since static

Use ff_msmpeg4_decode_init to initialize ff_msmp4_mb_i_vlc since static
VLC tables should only be initialized from one place.
This initializes/calculates more VLC tables than necessary for VC1 decoding,
but this is only done once and only a small overhead in time and space (maybe
30 kB) it seems not worth adding a separate function.

Originally committed as revision 20010 to svn://svn.ffmpeg.org/ffmpeg/trunk