fix a bug that was causing files with a blank video stream as the first channel to fail (flv files mainly)
fix a bug that was causing files with a blank video stream as the first channel to fail (flv files mainly)

joshrobo/pydub,cbelth/pyMusic,jiaaro/pydub,miguelgrinberg/pydub,sgml/pydub,lepinsk/pydub,Geoion/pydub