IMA ADPCM Stereo wav length half reduced

Hi.

I'm using libnyquist.
And found that while decoding wavs of type IMA ADPCM of two channels the length of the decoded file is reduced to half.
If I add 

```
        if (wavHeader.channel_count == 2)
          totalSamples *= 2;
```

On WavDecoder.cpp line 309 the problem seems to be fixed.

But I'm not sure if it is the right solution.

Thanks.