avcodec/aacdec: Fix runtime error: signed integer overflow: 2147483520 + 255 cannot be represented in type 'int'

Fixes: 1656/clusterfuzz-testcase-minimized-5900404925661184

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