message: Handle encoder_ == nullptr in VideoStreamEncoder::EncodeVideoFrame.
  This is to address a test failure seen on the msan bot(s).
  Tbr:
  Cr-Commit-Position: refs/heads/master@{#34233}
diff: @@ -1616,6 +1616,12 @@ void VideoStreamEncoder::EncodeVideoFrame(const VideoFrame& video_frame,
  if (encoder_failed_)
  return;
  + // It's possible that EncodeVideoFrame can be called after we've completed
  + // a Stop() operation. Check if the encoder_ is set before continuing.
  + // See: bugs.webrtc.org/12857
  + if (!encoder_)
  + return;
  +
  TraceFrameDropEnd();
  // Encoder metadata needs to be updated before encode complete callback.
  