Fixing uninitialized variable in file_audio_device.cc.

R=henrika@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17049004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6872 4adac7df-926f-26a2-2b94-8c16560cd09d