motionpixels: clip VLC codes.

Fixes invalid reads.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/motionpixels.c

@@ -180,6 +180,7 @@ static int mp_get_vlc(MotionPixelsContext *mp, GetBitContext *gb)
     int i;
 
     i = (mp->codes_count == 1) ? 0 : get_vlc2(gb, mp->vlc.table, mp->max_codes_bits, 1);
+    i = FFMIN(i, FF_ARRAY_ELEMS(mp->codes) - 1);
     return mp->codes[i].delta;
 }