fixed: AudioGeneratorMP3 reads uninitialized memory on first iteration

[RomanLut]

On the first iteration, AudioGeneratorMP3 is reading synth->pcm.samples[0][samplePtr] array which is not initialized.
May cause Pop sound due to garbage in array.

[earlephilhower]

Thanks for the contribution but going through the logic, it doesn't seem to be right. GetOneSample calls the mac-get-onens if there's nothing left buffered. That MAD call will fill in a series of samples and return an error or not. We return false here on error, indicating no data. If the onens call is successful we know that synth->pcm.samples[0][samplePtr (aka 0)];` is valid.

I also run the MP3 decode through valgrind as part of host tests and it catches things like reads to unwritten memory. It's not reported anything here.