libsepol/cil: Check for empty list when marking neverallow attributes

When marking a type attribute as used in a neverallow (to help determine
whether or not it should be expanded), check if the attribute's expression
list is empty (no attributes are associated with it) before iterating
over the list.

Signed-off-by: James Carter <jwcart2@gmail.com>
Acked-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsepol/cil/src/cil_post.c

@@ -1494,6 +1494,10 @@ static void __mark_neverallow_attrs(struct cil_list *expr_list)
 {
 	struct cil_list_item *curr;
 
+	if (!expr_list) {
+		return;
+	}
+
 	cil_list_for_each(curr, expr_list) {
 		if (curr->flavor == CIL_DATUM) {
 			if (FLAVOR(curr->data) == CIL_TYPEATTRIBUTE) {