Bug 1205339 - null out sIPCServingParent on teardown, to not assert on tab crash recovery. r=jesup

--HG--
extra : transplant_source : %90E8%8DQeX%C7m%084%A3%DF7%7D%D2%7B9%8E%15

dom/media/systemservices/MediaParent.cpp

@@ -545,6 +545,7 @@ DeallocPMediaParent(media::PMediaParent *aActor)
 {
   MOZ_ASSERT(sIPCServingParent == static_cast<Parent<PMediaParent>*>(aActor));
   delete sIPCServingParent;
+  sIPCServingParent = nullptr;
   return true;
 }