Ensure EncoderSink::OnBitrateAllocationUpdated not called after Stop

Bug: chromium:1143311, webrtc:12000
Change-Id: I149e960a4999442b289f4b3c576206cc6baf6f24
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/193063
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32605}

video/video_stream_encoder.cc

@@ -486,6 +486,7 @@ void VideoStreamEncoder::Stop() {
     }
     rate_allocator_ = nullptr;
     ReleaseEncoder();
+    encoder_ = nullptr;
     shutdown_event.Set();
   });
   shutdown_event.Wait(rtc::Event::kForever);