Prevent null-stream reconfigs on RTP extensions.

If a codec fails to set (e.g. there's no codec configured), this
prevents a stream reconfigure with an invalid config. Reconfiguring a
stream without correct codec settings causes a CHECK failure.

BUG=chromium:475116
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44219004

Cr-Commit-Position: refs/heads/master@{#9007}
1 file changed