A few places were relying on temporalIdx being signed. Fix to explicitly check for kNoTemporalIdx. TBR=pbos,stefan Review URL: https://webrtc-codereview.appspot.com/13939005 git-svn-id: http://webrtc.googlecode.com/svn/trunk@6669 4adac7df-926f-26a2-2b94-8c16560cd09d