avcodec/osq: avoid signed overflow in downsample path

Fixes: signed integer overflow: 865309950 * 256 cannot be represented in type 'int'
Fixes: 69191/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-6310214413385728

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 ed34b0c54e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/osq.c

@@ -320,7 +320,7 @@ static int do_decode(AVCodecContext *avctx, AVFrame *frame, int decorrelate, int
             cb->prev = prev;
 
             if (downsample)
-                dst[n] *= 256;
+                dst[n] *= 256U;
 
             dst[E] = dst[D];
             dst[D] = dst[C];