avcodec/tta: fix signed overflow in decorrelate

Fixes: signed integer overflow: 2079654542 - -139267653 cannot be represented in type 'int'
Fixes: 60811/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TTA_fuzzer-5915858409750528

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

libavcodec/tta.c

@@ -342,7 +342,7 @@ static int tta_decode_frame(AVCodecContext *avctx, AVFrame *frame,
             if (s->channels > 1) {
                 int32_t *r = p - 1;
                 for (*p += *r / 2; r > (int32_t*)p - s->channels; r--)
-                    *r = *(r + 1) - *r;
+                    *r = *(r + 1) - (unsigned)*r;
             }
             cur_chan = 0;
             i++;