some const

Originally committed as revision 11746 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/snow.c

@@ -4567,7 +4567,7 @@ static int decode_init(AVCodecContext *avctx)
     return 0;
 }
 
-static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, uint8_t *buf, int buf_size){
+static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, const uint8_t *buf, int buf_size){
     SnowContext *s = avctx->priv_data;
     RangeCoder * const c= &s->c;
     int bytes_read;