AudioTrack will mark 0-size buffers as consumed

This can happen in the first, non-codec-config output buffer
from MediaCodec, while decoding MP3 data.

This issue was discovered on Nexus 5's running both KitKat
Lollippop.
Merge pull request #208 from alexandrite/fix-mp3-buffer

AudioTrack will mark 0-size buffers as consumed