fix(agent): callback err

Author: dszakallas

lib/agent/index.js

@@ -124,7 +124,9 @@ Agent.prototype.stop = function (callback) {
       }
 
       if (counter >= agents.length) {
-        callback(error)
+        if (callback && typeof callback === 'function') {
+          callback(error)
+        }
       } else {
         counter++
       }