avcodec/vmixdec: Fix signed integer overflow in dc

Fixes: signed integer overflow: 2147483181 + 1024 cannot be represented in type 'int'
Fixes: 61117/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VMIX_fuzzer-5387692433866752

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vmixdec.c

@@ -148,7 +148,7 @@ static int decode_dcac(AVCodecContext *avctx,
                     ac_run = get_ue_golomb_long(ac_gb);
             }
 
-            block[0] = (dc << lshift) + add;
+            block[0] = ((unsigned)dc << lshift) + (unsigned)add;
             s->idsp.idct_put(dst + x, linesize, block);
         }