avcodec/aacdec_fixed: Fix runtime error: signed integer overflow: -2147483648 * -1 cannot be represented in type 'int'

Fixes: 1825/clusterfuzz-testcase-minimized-6002833050566656

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

libavcodec/aacdec_fixed.c

@@ -187,7 +187,7 @@ static void subband_scale(int *dst, int *src, int scale, int offset, int len)
         round = 1U << (s-1);
         for (i=0; i<len; i++) {
             out = (int)((int64_t)((int64_t)src[i] * c + round) >> s);
-            dst[i] = out * ssign;
+            dst[i] = out * (unsigned)ssign;
         }
     }
 }