[settings] Use list_first_entry() when unregistering child settings

Unregistering a child settings block can have almost arbitrary
effects, due to the call to apply_settings().  Avoid potentially
dereferencing a stale pointer by using list_first_entry() rather than
list_for_each_entry_safe() to iterate over the list of child settings.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

  */
 void unregister_settings ( struct settings *settings ) {
 	struct settings *child;
-	struct settings *tmp;
 
 	/* Unregister child settings */
-	list_for_each_entry_safe ( child, tmp, &settings->children, siblings ) {
+	while ( ( child = list_first_entry ( &settings->children,
+					     struct settings, siblings ) ) ) {
 		unregister_settings ( child );
 	}