fix exception when receiving REFER but dial task ended (#1353)

lib/session/call-session.js

@@ -2681,7 +2681,7 @@ Duration=${duration} `
    */
   _onRefer(req, res) {
     const task = this.currentTask;
-    const sd = task.sd;
+    const sd = task?.sd;
     if (task && TaskName.Dial === task.name && sd && task.referHook) {
       task.handleRefer(this, req, res);
     }