[Bug Report] pull down 当下拉到较大距离的时候，手指点一下屏幕，很大概率会出现页面卡主不动，然后上面的提示由‘释放更新’到‘下拉刷新’

### Mint UI version
2.2.13

### OS/Browsers version
iOS，Android

### Vue version
2.5.2

### Reproduction Link
[http://elemefe.github.io/mint-ui/#/pull-down](http://elemefe.github.io/mint-ui/#/pull-down)

### Steps to reproduce
重现链接用的官网的，用这个就能出现这个问题。
重现步骤：
下拉刷新，按住不动，拉到不能再拉为止，然后按住不动，用另一个手指点一下屏幕。

### What is Expected?
手指点一下屏幕对下拉刷新的状态应该没有影响，放开手指后应该刷新页面

### What is actually happening?
一个手指不动，另一个手指点击屏幕后，下拉的状态由 ‘释放更新’到‘下拉刷新’ 。 手指全部松开后，页面静止，上面留有大片空白，上面显示 ‘下拉刷新’

Start with the pull-down reproduction page at http://elemefe.github.io/mint-ui/#/pull-down and follow the reported two-finger gesture sequence on iOS and Android. Verify the pull-down state after one finger taps while the other holds at the maximum distance; done means releasing both fingers refreshes normally without a stuck page or blank area.

Written by the indexing model from the issue text.