Return false if PeerConnection::GetStats() is called on invalid tracks

webrtc/api/peerconnection.cc

Index: webrtc/api/peerconnection.cc
diff --git a/webrtc/api/peerconnection.cc b/webrtc/api/peerconnection.cc
index 830c301a6d9cf0090870e3a06bca4ce23835ec0c..a13bcda7a79bde4c649830064fdd968e61ecc481 100644
--- a/webrtc/api/peerconnection.cc
+++ b/webrtc/api/peerconnection.cc
@@ -886,6 +886,9 @@ bool PeerConnection::GetStats(StatsObserver* observer,
   }
 
   stats_->UpdateStats(level);
+  if (!stats_->IsValidTrack(track)) {

[Taylor Brandstetter, 2016/11/03 22:47:01]

May be worth logging a warning here.

Also, you could add a comment explaining "The StatsCollector is used to tell if
a track is valid because it may remember tracks that the PeerConnection
previously removed." I didn't realize this at first.

Oh, and one more thing; a null track *should not* be an error. That should just
equate to "no track selector specified, gather all stats". So you could change
the condition to:

"if (track && !stats_->IsValidTrack(track))"

[Zhi Huang, 2016/11/03 23:17:27]

I realized that the nullptr is not an error. The nullptr condition was inside
the IsValidTrack() before. I'll move it here.

Done.

+    return false;
+  }
   signaling_thread()->Post(RTC_FROM_HERE, this, MSG_GETSTATS,
                            new GetStatsMsg(observer, track));
   return true;