Prevent null-stream reconfigs on RTP extensions.
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9007}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3c3f6460646183914629e5dab8ae5fcede4f0e80

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