Avoid crashes when closing peer connection client.

// FREEBIE

Signal/src/call/PeerConnectionClient.swift

@@ -339,7 +339,6 @@ class PeerConnectionClient: NSObject, RTCPeerConnectionDelegate, RTCDataChannelD
 
         return setRemoteSessionDescription(remoteDescription)
             .then(on: PeerConnectionClient.signalingQueue) {
-                assert(self.peerConnection != nil)
                 return self.negotiateAnswerSessionDescription(constraints: constraints)
         }
     }
@@ -369,6 +368,12 @@ class PeerConnectionClient: NSObject, RTCPeerConnectionDelegate, RTCDataChannelD
         return Promise { fulfill, reject in
             assertOnSignalingQueue()
 
+            guard self.peerConnection != nil else {
+                Logger.debug("\(self.TAG) \(#function) Ignoring obsolete event in terminated client")
+                reject(NSError(domain:"Obsolete client", code:0, userInfo:nil))
+                return
+            }
+
             Logger.debug("\(self.TAG) negotiating answer session.")
 
             peerConnection.answer(for: constraints, completionHandler: { (sdp: RTCSessionDescription?, error: Error?) in