avcodec/mjpegdec: Fix integer overflow in DC dequantization

Fixes: runtime error: signed integer overflow: -65535 * 65312 cannot be represented in type 'int'
Fixes: 4900/clusterfuzz-testcase-minimized-5769019744321536

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

libavcodec/mjpegdec.c

@@ -715,7 +715,7 @@ static int decode_block(MJpegDecodeContext *s, int16_t *block, int component,
         av_log(s->avctx, AV_LOG_ERROR, "error dc\n");
         return AVERROR_INVALIDDATA;
     }
-    val = val * quant_matrix[0] + s->last_dc[component];
+    val = val * (unsigned)quant_matrix[0] + s->last_dc[component];
     val = av_clip_int16(val);
     s->last_dc[component] = val;
     block[0] = val;