Fix bugs introduced by https://code.google.com/p/webrtc/source/detail?r=6667 .

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