Call OnTransportFeedback just when feedback_observer exist.

webrtc/voice_engine/channel.cc

Index: webrtc/voice_engine/channel.cc
diff --git a/webrtc/voice_engine/channel.cc b/webrtc/voice_engine/channel.cc
index 97132995ba1fe8723d54240ee9d9892fa652b7b7..7be47d8222f03637a6bc28209692c119c45b069e 100644
--- a/webrtc/voice_engine/channel.cc
+++ b/webrtc/voice_engine/channel.cc
@@ -211,7 +211,8 @@ class TransportFeedbackProxy : public TransportFeedbackObserver {
   void OnTransportFeedback(const rtcp::TransportFeedback& feedback) override {
     RTC_DCHECK(network_thread_.CalledOnValidThread());
     rtc::CritScope lock(&crit_);
-    feedback_observer_->OnTransportFeedback(feedback);
+    if (feedback_observer_)
+      feedback_observer_->OnTransportFeedback(feedback);
   }
   std::vector<PacketInfo> GetTransportFeedbackVector() const override {
     RTC_NOTREACHED();