wavarc: fix decoding of 16-bit sample

[ami_stuff]

docompresses correctly with wa32.exe

./ffmpeg -i 16_33_M_C1.WA -y out.wav
ffmpeg version N-109761-gc5aeb8f4db 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
Input #0, wavarc, from '16_33_M_C1.WA':
  Duration: N/A, start: 0.000000, bitrate: N/A
  Stream #0:0: Audio: wavarc (1DIF / 0x46494431), 33333 Hz, mono, s16p
Stream mapping:
  Stream #0:0 -> #0:0 (wavarc (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf59.38.100
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 33333 Hz, mono, s16, 533 kb/s
    Metadata:
      encoder         : Lavc59.63.100 pcm_s16le
size=     651kB time=00:00:09.99 bitrate= 533.4kbits/s speed= 454x    
video:0kB audio:651kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.011700%