Avoid crashes on macOS with context menus

* src/nsmenu.m ([EmacsMenu menuNeedsUpdate:]): Avoid crashes with
context menus.  (Bug#63495)

src/nsmenu.m

@@ -477,6 +477,14 @@ - (instancetype)initWithTitle: (NSString *)title
    call to ns_update_menubar.  */
 - (void)menuNeedsUpdate: (NSMenu *)menu
 {
+
+  /* The context menu is built and then displayed, as opposed to the
+     top-menu, which is partially built and then updated and filled in
+     when it's time to display it.  Therefore, we don't call
+     ns_update_menubar if a context menu is active. */
+  if (context_menu_value != 0)
+    return;
+
 #ifdef NS_IMPL_GNUSTEP
   static int inside = 0;
 #endif