fix after r6472 in rtp_sender, comparison between signed and unsigned integer expressions.

BUG=N/A
R=pwestin@webrtc.org, wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15909004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6539 4adac7df-926f-26a2-2b94-8c16560cd09d