avcodec/flacdec: Fix signed integre overflow

Fixes: signed integer overflow: 3011809745540902265 + 6323452730883571725 cannot be represented in type 'long'
Fixes: 45982/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-6687553022722048

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

libavcodec/flacdec.c

@@ -513,7 +513,7 @@ static int decode_subframe_lpc_33bps(FLACContext *s, int64_t *decoded,
     for (i = pred_order; i < s->blocksize; i++, decoded++) {
         int64_t sum = 0;
         for (j = 0; j < pred_order; j++)
-            sum += (int64_t)coeffs[j] * decoded[j];
+            sum += (int64_t)coeffs[j] * (uint64_t)decoded[j];
         decoded[j] = residual[i] + (sum >> qlevel);
     }