wav: fix decoding of 16-bit RIFX

[ami_stuff]

support for RIFX WAV format has been added in 00d7555f3468193a761c0534df6742646b15829c (ticket #1978), but attached 16-bit RIFX file created with SoX decodes only to noise

Audacity for example is able to decode it correctly.

FFmpeg should either decode 16-bit RIFX files correctly or reject them.

24-bit and 32-bit RIFX files created with SoX doesn't decode at all with FFmpeg because of:

[wav @ 0x7fe8b0000cc0] WAVEFORMATEX support for RIFX files is not implemented.
Update your FFmpeg version to the newest one from Git. If the problem still occurs,
it means that your file has a feature which has not been implemented.

./ffmpeg -i 16bit_rifx.wav -y out.wav
ffmpeg version N-109867-g5247dab6b1 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, wav, from '16bit_rifx.wav':
  Duration: 00:00:10.00, bitrate: 533 kb/s
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 33333 Hz, 1 channels, s16, 533 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
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=     651kB time=00:00:09.95 bitrate= 535.9kbits/s speed=1.67e+03x    
video:0kB audio:651kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.011700%