Bug 993780 - Ignore calls to SetSignalingState_m once PC is in close. r=jib,rjesup

media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.cpp

@@ -1753,7 +1753,8 @@ void
 PeerConnectionImpl::SetSignalingState_m(PCImplSignalingState aSignalingState)
 {
   PC_AUTO_ENTER_API_CALL_NO_CHECK();
-  if (mSignalingState == aSignalingState) {
+  if (mSignalingState == aSignalingState ||
+      mSignalingState == PCImplSignalingState::SignalingClosed) {
     return;
   }