check for handler presence in v-on reset (#366)

Author: Evan You

src/directives/on.js

@@ -46,7 +46,9 @@ module.exports = {
         var el = this.iframeBind
             ? this.el.contentWindow
             : this.el
-        el.removeEventListener(this.arg, this.handler)
+        if (this.handler) {
+            el.removeEventListener(this.arg, this.handler)
+        }
     },
 
     unbind: function () {