Fix uninitialized value warning in rtp_payload_registry and make sure we return an error if the payload type isn't registered.

R=pbos@webrtc.org, xians@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1782004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4321 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed