Unbreak NDEBUG compile by RTC_UNUSED()ing an assert()d variable.

TBR=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/19659004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6357 4adac7df-926f-26a2-2b94-8c16560cd09d