Empty main menu causes panic

[Atomotron]

Setting the window's main menu to an empty main menu will cause a panic on glfw.
This line will cause a panic:
w.SetMainMenu(fyne.NewMainMenu())
The panic is:
panic: runtime error: index out of range [0] with length 0

This happens with the glfw driver on the develop branch.

The panic is caused by a line in this file. The line begins with,

if menus.Items[0].Items[len(menus.Items[0].Items)-1].Label

This assumes that menus.Items has an entry at index zero.

[andydotxyz]

The fix is on develop branch for testing