Fix crash on removing Windows PopupMenu submenu

[kitbdev]

When freeing a node that has a native Windows PopupMenu with a submenu, it crashes.
This happens since 4.3 as far as I can tell.

Reproduction example script:

func _ready() -> void:
	var submenu := PopupMenu.new()
	submenu.add_item("b")
	var menu := PopupMenu.new()
	menu.prefer_native_menu = true
	menu.add_submenu_node_item("a", submenu)
	add_child(menu)
	queue_free() # also happens on exit without this line

p_submenu_rid does not need to be valid, so I put the code that uses it into the if statement.

[Repiteo]

Thanks!