Fix: Crash in interface layout when using custom nodes

`node_socket_add_tooltip_in_node_editor` doesn't set a free function.
but uiLayoutSetTooltipFunc didn't check on it. Documentation states
that the free_arg parameter is optional.

Pull Request: https://projects.blender.org/blender/blender/pulls/122115

source/blender/editors/interface/interface_layout.cc

@@ -6039,7 +6039,7 @@ void uiLayoutSetTooltipFunc(uiLayout *layout,
     }
   }
 
-  if (!arg_used) {
+  if (free_arg != nullptr && !arg_used) {
     /* Free the original copy of arg in case the layout is empty. */
     free_arg(arg);
   }