avcodec/truemotion2: Fix integer overflow in tm2_decode_blocks()

Fixes: signed integer overflow: 255 + 2147483634 cannot be represented in type 'int'
Fixes: 13472/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5712444142387200

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 0ad0533e91)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/truemotion2.c

@@ -824,7 +824,7 @@ static int tm2_decode_blocks(TM2Context *ctx, AVFrame *p)
     dst = p->data[0];
     for (j = 0; j < h; j++) {
         for (i = 0; i < w; i++) {
-            int y = Y[i], u = U[i >> 1], v = V[i >> 1];
+            unsigned y = Y[i], u = U[i >> 1], v = V[i >> 1];
             dst[3*i+0] = av_clip_uint8(y + v);
             dst[3*i+1] = av_clip_uint8(y);
             dst[3*i+2] = av_clip_uint8(y + u);