BMenuItem: Remove ourselves from the super menu on destruct.

This fixes the (intermittently) crashing test added in the previous commit,
and should also fix #12024 and #14348.

Note that this is a slight behavioral departure from BeOS, though since
BeOS crashed when this was done previously, it shouldn't cause any
other problems.

Change-Id: I90b6132ff7741b8d6cb601375a9b11fc3ffacb40
Reviewed-on: https://review.haiku-os.org/541
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/user/interface/MenuItem.dox

@@ -115,7 +115,9 @@
 	\fn BMenuItem::~BMenuItem()
 	\brief Destructor.
 
-	Also frees the memory used by the label or submenu.
+	If this item is attached to a menu, it will be removed from it.
+
+	Also destroys the label and submenu.
 
 	\since BeOS R3
 */

src/kits/interface/MenuItem.cpp

@@ -181,6 +181,9 @@ BMenuItem::Archive(BMessage* data, bool deep) const
 
 BMenuItem::~BMenuItem()
 {
+	if (fSuper != NULL)
+		fSuper->RemoveItem(this);
+
 	free(fLabel);
 	delete fSubmenu;
 }