ape: fix decoding of monkey's audio 3.80 8-bit files

[ami_stuff]

./ffmpeg -i 8m_3_80_normal.ape -y out.wav
ffmpeg version N-112010-g1a87a9d82a Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 11 (Ubuntu 11.4.0-1ubuntu1~22.04)
  configuration: 
  libavutil      58. 19.100 / 58. 19.100
  libavcodec     60. 26.100 / 60. 26.100
  libavformat    60. 11.100 / 60. 11.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9. 11.100 /  9. 11.100
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
Input #0, ape, from '8m_3_80_normal.ape':
  Duration: 00:00:10.50, start: 0.000000, bitrate: 123 kb/s
  Stream #0:0: Audio: ape (APE  / 0x20455041), 44100 Hz, mono, u8p
Stream mapping:
  Stream #0:0 -> #0:0 (ape (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf60.11.100
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
    Metadata:
      encoder         : Lavc60.26.100 pcm_s16le
[out#0/wav @ 0x55ca8edd1140] video:0kB audio:904kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.008422%
size=     904kB time=00:00:10.44 bitrate= 709.1kbits/s speed= 308x