Update demuxer.coffe

correct problems with seeking (flac file). This my first code on coffe. Correct if i does error)))
To test this code use http://master255.no-ip.org/?file=/res/Музыка/B/Benny Benassi/Benny Benassi - Cinema (Skrillex Remix) (featuring Gary Go) (Bonus Track).flac
I know this is temporarily bugfix, but this correct error at now moment.