Use correct rtp header size for FEC packets.
This is needed as of r2489 which introduced the header extension.

This also is a fix for issue 701.
Review URL: https://webrtc-codereview.appspot.com/708005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2537 4adac7df-926f-26a2-2b94-8c16560cd09d
4 files changed