Prevent asserting on unset start bitrate.

Quick fix for crash in WebRtcBrowserTest.NegotiateOfferWithBLine.
Follow-up CL from stefan@ that changes this path is under review.

BUG=1788,1667
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/51419005

Cr-Commit-Position: refs/heads/master@{#8823}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8823 4adac7df-926f-26a2-2b94-8c16560cd09d