[Bug Report] [navMenu] click el-submenu trigger childMenu layer pop again

### Element UI version
2.4.6

### OS/Browsers version
windows10/chrome 68.0.3440.106

### Vue version
2.5.2

### Reproduction Link
[https://jsfiddle.net/api/post/library/pure/](https://jsfiddle.net/api/post/library/pure/)

### Steps to reproduce
English:
at the demo link, choose the anyone of submenu, the submenu layer pop again, accidental,need exprience more times

中文：
在demo链接的这个例子中，在菜单是collapse状态时，选中没有子菜单的一级菜单，然后再选中el-submenu中的某一项，子菜单弹层会消失再弹出，这个现象是偶发，需要来回点击多次才能发现。

### What is Expected?
English:menu in collapse,when i choose submenu, the submenu layer disappear, and don't pop again.

中文：选中子菜单后，子菜单弹出层消失，不再自动弹出，需要再次人为点击时才能再次弹出。

### What is actually happening?
English:menu in collapse, when i click menu, submenu pop at right of menu, choose el-meny-item, and then i choose submenu,the submenu disappear, and pop again!

中文：点击el-submenu中的某一项，子菜单会消失，再弹出，实际上，他不应该再次弹出。