Fix missing condition

Author: sonologico

src/main/java/com/pusher/client/user/impl/InternalUser.java

@@ -143,7 +143,9 @@ private void onSigninSuccess(PusherEvent event) {
     }
 
     private void disconnect() {
-        channelManager.unsubscribeFrom(serverToUserChannel.getName());
+        if (serverToUserChannel.isSubscribed()) {
+            channelManager.unsubscribeFrom(serverToUserChannel.getName());
+        }
         userId = null;
     }