vp8: fix signed overflows

In addition to avoiding undefined behaviour, an unsigned type
makes more sense for packing multiple 8-bit values.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/vp8.c

@@ -919,7 +919,8 @@ void intra_predict(VP8Context *s, uint8_t *dst[3], VP8Macroblock *mb,
                    int mb_x, int mb_y)
 {
     AVCodecContext *avctx = s->avctx;
-    int x, y, mode, nnz, tr;
+    int x, y, mode, nnz;
+    uint32_t tr;
 
     // for the first row, we need to run xchg_mb_border to init the top edge to 127
     // otherwise, skip it if we aren't going to deblock
@@ -948,7 +949,7 @@ void intra_predict(VP8Context *s, uint8_t *dst[3], VP8Macroblock *mb,
         // from the top macroblock
         if (!(!mb_y && avctx->flags & CODEC_FLAG_EMU_EDGE) &&
             mb_x == s->mb_width-1) {
-            tr = tr_right[-1]*0x01010101;
+            tr = tr_right[-1]*0x01010101u;
             tr_right = (uint8_t *)&tr;
         }