bonk: fix decoding of lossless joint stereo mode

[ami_stuff]

looks like regression

./ffmpeg -i 22k_joint_lossless.bonk -y out.wav
ffmpeg version N-109341-g6b368bcb85 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: --enable-libopenjpeg
  libavutil      57. 43.100 / 57. 43.100
  libavcodec     59. 54.100 / 59. 54.100
  libavformat    59. 34.102 / 59. 34.102
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 51.100 /  8. 51.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, bonk, from '22k_joint_lossless.bonk':
  Duration: 00:00:20.00, bitrate: 465 kb/s
  Stream #0:0: Audio: bonk, 22050 Hz, 2 channels, s16p
Stream mapping:
  Stream #0:0 -> #0:0 (bonk (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf59.34.102
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 22050 Hz, stereo, s16, 705 kb/s
    Metadata:
      encoder         : Lavc59.54.100 pcm_s16le
Error while decoding stream #0:0: Invalid data found when processing input
    Last message repeated 14 times
size=    1432kB time=00:00:16.60 bitrate= 706.7kbits/s speed= 118x    
video:0kB audio:1432kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.005319%

./ffmpeg -i 22k_joint_lossless.bonk -y out.wav
ffmpeg version N-109064-gc124981b79 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: 
  libavutil      57. 42.100 / 57. 42.100
  libavcodec     59. 52.101 / 59. 52.101
  libavformat    59. 34.101 / 59. 34.101
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 50.100 /  8. 50.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, bonk, from '22k_joint_lossless.bonk':
  Duration: 00:00:20.00, bitrate: 465 kb/s
  Stream #0:0: Audio: bonk, 22050 Hz, 2 channels, s16p
Stream mapping:
  Stream #0:0 -> #0:0 (bonk (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf59.34.101
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 22050 Hz, stereo, s16, 705 kb/s
    Metadata:
      encoder         : Lavc59.52.101 pcm_s16le
size=    1723kB time=00:00:19.99 bitrate= 705.9kbits/s speed= 124x    
video:0kB audio:1723kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.004422%