avcodec/aacdec_fixed: Fix integer overflow in apply_independent_coupling_fixed()

I was not able to reproduce this, this fix is based on just the fuzzer log.
Fixes: 4959/clusterfuzz-testcase-minimized-6035350934781952

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

libavcodec/aacdec_fixed.c

@@ -417,7 +417,7 @@ static void apply_independent_coupling_fixed(AACContext *ac,
     int i, c, shift, round, tmp;
     const int gain = cce->coup.gain[index][0];
     const int *src = cce->ch[0].ret;
-    int *dest = target->ret;
+    unsigned int *dest = target->ret;
     const int len = 1024 << (ac->oc[1].m4ac.sbr == 1);
 
     c = cce_scale_fixed[gain & 7];