Fixed deference of NULL pointer in motionpixels decoder.

Some of the arguments given to init_vlc() come from the stream
and can be corrupted.
Signed-off-by: Janne Grunau <janne-libav@jannau.net>

libavcodec/motionpixels.c

@@ -278,7 +278,8 @@ static int mp_decode_frame(AVCodecContext *avctx,
     if (sz == 0)
         goto end;
 
-    init_vlc(&mp->vlc, mp->max_codes_bits, mp->codes_count, &mp->codes[0].size, sizeof(HuffCode), 1, &mp->codes[0].code, sizeof(HuffCode), 4, 0);
+    if (init_vlc(&mp->vlc, mp->max_codes_bits, mp->codes_count, &mp->codes[0].size, sizeof(HuffCode), 1, &mp->codes[0].code, sizeof(HuffCode), 4, 0))
+        goto end;
     mp_decode_frame_helper(mp, &gb);
     free_vlc(&mp->vlc);