Fixed potential crash if rtp packet history is completely full.

Also performance enhanecement in rtp_sender (don't lookup if kDontStore)

BUG=4171
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/39759004

Cr-Commit-Position: refs/heads/master@{#8226}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8226 4adac7df-926f-26a2-2b94-8c16560cd09d
4 files changed