Re-add missing return in RegisterExternalDecoder.

Breaks waterfall due to possible null-pointer dereferences.

BUG=webrtc:5249
TBR=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1483623002 .

Cr-Commit-Position: refs/heads/master@{#10824}

webrtc/modules/video_coding/video_receiver.cc

@@ -245,6 +245,7 @@ void VideoReceiver::RegisterExternalDecoder(VideoDecoder* externalDecoder,
     // Make sure the VCM updates the decoder next time it decodes.
     _decoder = NULL;
     RTC_CHECK(_codecDataBase.DeregisterExternalDecoder(payloadType));
+    return;
   }
   _codecDataBase.RegisterExternalDecoder(externalDecoder, payloadType,
                                          internalRenderTiming);