wavarc: fix decoding of compression level 0

[ami_stuff]

docompresses correctly with wa32.exe

./ffmpeg -i 8_33_M_C0.WA -y out.wav
ffmpeg version N-109763-g3b3a321fed Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: 
  libavutil      57. 44.100 / 57. 44.100
  libavcodec     59. 63.100 / 59. 63.100
  libavformat    59. 38.100 / 59. 38.100
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 56.100 /  8. 56.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
[wavarc @ 0x55d583d45440] Format wavarc detected only with low score of 1, misdetection possible!
[wavarc @ 0x55d583d45440] Failed to open codec in avformat_find_stream_info
    Last message repeated 1 times
[wavarc @ 0x55d583d45440] Could not find codec parameters for stream 0 (Audio: wavarc (0CPY / 0x59504330), 33333 Hz, 0 channels, u8p): unspecified number of channels
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, wavarc, from '8_33_M_C0.WA':
  Duration: N/A, start: 0.000000, bitrate: N/A
  Stream #0:0: Audio: wavarc (0CPY / 0x59504330), 33333 Hz, mono, u8p
Stream mapping:
  Stream #0:0 -> #0:0 (wavarc (native) -> pcm_s16le (native))
Error while opening decoder for input stream #0:0 : Invalid data found when processing input