avcodec/flacdec: Fix overflow in "33bit" decorrelate

Fixes: signed integer overflow: 538976288 - -9223372036854775808 cannot be represented in type 'long'
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-6275845531238400

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 35e6960a6b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/flacdec.c

@@ -706,10 +706,10 @@ static void decorrelate_33bps(int ch_mode, int32_t **decoded, int64_t *decoded_3
     int i;
     if (ch_mode == FLAC_CHMODE_LEFT_SIDE ) {
         for (i = 0; i < len; i++)
-           decoded[1][i] = decoded[0][i] - decoded_33bps[i];
+           decoded[1][i] = decoded[0][i] - (uint64_t)decoded_33bps[i];
     } else if (ch_mode == FLAC_CHMODE_RIGHT_SIDE ) {
         for (i = 0; i < len; i++)
-           decoded[0][i] = decoded[1][i] + decoded_33bps[i];
+           decoded[0][i] = decoded[1][i] + (uint64_t)decoded_33bps[i];
     } else if (ch_mode == FLAC_CHMODE_MID_SIDE ) {
         for (i = 0; i < len; i++) {
             uint64_t a = decoded[0][i];