avcodec/osq: use unsigned for decorrelation

Fixes: signed integer overflow: 1205469696 + 1901074655 cannot be represented in type 'int'
Fixes: 70773/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-5419594888577024

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

libavcodec/osq.c

@@ -342,7 +342,7 @@ static int do_decode(AVCodecContext *avctx, AVFrame *frame, int decorrelate, int
 
             if (nb_channels == 2 && ch == 1) {
                 if (decorrelate)
-                    dst[n] += s->decode_buffer[0][OFFSET+n];
+                    dst[n] += (unsigned)s->decode_buffer[0][OFFSET+n];
             }
 
             if (downsample)