Fix calculation for the estimated total samples of the encoded file

I noticed that ever since the merge of audioencoder binary addons the FLAC encoder didn't produce playable/valid files but it worked fine before. So I hunted down the only real change to the old code and that is the calculation of the estimated total samples of the encoded file. See https://github.com/Montellese/audioencoder.flac/commit/71ffcf71c679bca4e2c30cbc23a040ce56abad71 for the change and a detailed comment from me. I checked some files and these are the values of iTrackLength:
* 3m 05s = 185s: 32'725'728
* 2m 35s = 155s: 27'461'952
* 3m 23s = 203s: 35'849'184

So it looks like if that value is divided by `4` and then by the number of seconds of the track you end up at around `44100` which is the expected sample rate. So basically the calculation is
```
iTrackLength = <sample rate> * 4 * <total seconds>
```
so dividing `iTrackLength` by 4 will give the estimated total samples.

Since I can only test on win32 and @jmarshallnz made that change while developing on OSX and no other audioencoder makes use of `iTrackLength` maybe this is a platform-specific issue. So I'd be grateful if someone could test this on linux and OSX.