libfdk-aacdec: Correct buffer_size parameter

The timeDataSize argument to aacDecoder_DecodeFrame() seems undocumented
and until 2016 04 (203e3f28fbebec7011342017fafc2a0bda0ce530) unused.
After that commit libfdk-aacdec interprets it as size in sample units
and memsets that on error.

FFmpeg as well as others (like GStreamer) did interpret it as size in
bytes.

Fixes: 1442/clusterfuzz-testcase-minimized-4540199973421056 (This requires recent libfdk to reproduce)

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Martin Storsjö <martin@martin.st>
