Fix uninitialized value in DtlsTransport and TransportDescription.

BUG=crbug/390304
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21779004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6577 4adac7df-926f-26a2-2b94-8c16560cd09d