[bug report] when the table component uses the total row at the end of the table and there is a horizontal scroll bar, the scroll bar of the fixed column fails

### Element UI version
2.13.0

### OS/Browsers version
chrome 78.0.3904.70

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/moerm/pen/povWRQq?&editable=true

### Steps to reproduce
1、table组件创建一个有fixed属性的首列，填充列直至出现横向滚动条。
2、拖动滚动条，非fixed的表格的滚动条可正常滚动
3、fixed的列中的滚动条无法滚动

### What is Expected?
fixed列中滚动条可以正常使用

### What is actually happening?
fixed列中滚动条无法使用