Ensure a stale peerConnectionClient doesn't call any methods on the CallService

Nil delegates when terminating call

// FREEBIE

Signal/src/call/CallService.swift

@@ -885,6 +885,7 @@ fileprivate let timeoutSeconds = 60
         assertOnSignalingQueue()
         Logger.debug("\(TAG) in \(#function)")
 
+        peerConnectionClient?.delegate = nil
         peerConnectionClient?.terminate()
 
         peerConnectionClient = nil

Signal/src/call/PeerConnectionClient.swift

@@ -53,7 +53,7 @@ class PeerConnectionClient: NSObject, RTCPeerConnectionDelegate, RTCDataChannelD
     }
 
     // Delegate is notified of key events in the call lifecycle.
-    private weak var delegate: PeerConnectionClientDelegate!
+    public weak var delegate: PeerConnectionClientDelegate!
 
     // Connection