rka: fix decoding of 8-bit lossless

[ami_stuff]

rkau.exe decompresses the file correctly

ideally the output should be identical to:

​https://trac.ffmpeg.org/raw-attachment/ticket/10174/8_33_M_C0.WA

./ffmpeg -i 8_33_m_l1.rka -y out.wav
ffmpeg version N-109858-g7c41a083b7 Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: 
  libavutil      58.  1.100 / 58.  1.100
  libavcodec     60.  2.100 / 60.  2.100
  libavformat    60.  2.100 / 60.  2.100
  libavdevice    60.  0.100 / 60.  0.100
  libavfilter     9.  1.100 /  9.  1.100
  libswscale      7.  0.100 /  7.  0.100
  libswresample   4.  9.100 /  4.  9.100
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, rka, from '8_33_m_l1.rka':
  Duration: 00:00:10.00, start: 0.000000, bitrate: 103 kb/s
  Stream #0:0: Audio: rka, 33333 Hz, 1 channels, u8p
Stream mapping:
  Stream #0:0 -> #0:0 (rka (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Error while decoding stream #0:0: Invalid data found when processing input
    Last message repeated 2 times
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf60.2.100
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 33333 Hz, mono, s16, 533 kb/s
    Metadata:
      encoder         : Lavc60.2.100 pcm_s16le
size=       0kB time=-577014:32:22.77 bitrate=N/A speed=N/A    ed=N/A    
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
Conversion failed!

[Elon Musk]

Fixed, currently outputs few samples extra sometimes.