Transfer 组件在render后，马上unmount会报setState错误 (#2206)

* Transfer 组件在render后，马上unmount会报setState错误

如题

* _t => timer

components/transfer/list.jsx

@@ -54,13 +54,17 @@ export default class TransferList extends React.Component {
   }
 
   componentDidMount() {
-    setTimeout(() => {
+    this.timer = setTimeout(() => {
       this.setState({
         mounted: true,
       });
     }, 0);
   }
 
+  componentWillUnmount() {
+    clearTimeout(this.timer);
+  }
+
   shouldComponentUpdate(...args) {
     return PureRenderMixin.shouldComponentUpdate.apply(this, args);
   }