Sending empty RTP packets caused a crash when using a generic codec instead of VP8. This fix moves handling of empty RTP packets out of ReceiveVp8Codec and into ParseRtpPacket. BUG= R=stefan@webrtc.org Review URL: https://webrtc-codereview.appspot.com/2185004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@4701 4adac7df-926f-26a2-2b94-8c16560cd09d