TransmitMixer: Check GetSendCodec return value.

BUG=b/62909493

Review-Url: https://codereview.webrtc.org/2973083002
Cr-Original-Commit-Position: refs/heads/master@{#18975}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 950c1c908c46d10f3de6081e52718a304a199b61
2 files changed