[bug report] elmenu subcomponent recursively reports the maximum call stack size exceeded

### Element UI version
0.1.0

### OS/Browsers version
window 7/chrome 77

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/nzf7s4vh/

### Steps to reproduce
收起状态鼠标移入有子菜单的项，控制台报错如下：
Maximum call stack size exceeded

### What is Expected?
不报错

### What is actually happening?
报错