Bug 1223655 - Only check for track end if track exists in MediaEngineDefaultAudioSource. r=jesup

--HG--
extra : commitid : 9cp03yDbKSh
extra : rebase_source : 086bfa9a3a2996068cd60c8f75cc1b064160a94a
extra : histedit_source : 56ae9ea5451aba65913eb2a8af7e139a15fb1910

dom/media/webrtc/MediaEngineDefault.h

@@ -136,7 +136,8 @@ public:
                           TrackID aId,
                           StreamTime aDesiredTime) override
   {
-    NS_WARN_IF_FALSE(aDesiredTime <= aSource->GetEndOfAppendedData(aId),
+    NS_WARN_IF_FALSE(!aSource->FindTrack(aId) ||
+                     aDesiredTime <= aSource->GetEndOfAppendedData(aId),
                      "MediaEngineDefaultAudioSource data underrun");
   }