avcodec/sheervideo: fix prediction for ybyr format

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/sheervideo.c

@@ -1222,7 +1222,7 @@ static void decode_ybyr(AVCodecContext *avctx, AVFrame *p, GetBitContext *gb)
             dst_v[x / 2] = get_bits(gb, 8);
         }
     } else {
-        int pred[4] = { -125, 128, 128, 0 };
+        int pred[4] = { -128, 128, 128, 0 };
 
         for (x = 0; x < avctx->width; x += 2) {
             int y1, y2, u, v;