avcodec/utvideodec: Fix integer overflow in decode_plane()

Fixes: signed integer overflow: 2147483594 + 142 cannot be represented in type 'int'
Fixes: 20492/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_UTVIDEO_fuzzer-5658568101724160

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 876cfa67f3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utvideodec.c

@@ -109,7 +109,7 @@ static int decode_plane(UtvideoContext *c, int plane_no,
                 for (i = 0; i < width * step; i += step) {
                     pix = fsym;
                     if (use_pred) {
-                        prev += pix;
+                        prev += (unsigned)pix;
                         pix   = prev;
                     }
                     dest[i] = pix;