slist: Fix free_all function

lib/types/slist.c

@@ -102,5 +102,14 @@ void
 slist_free_all (SList    *list,
                 FreeFunc  func)
 {
-  slist_foreach (list, (ForEachFunc) slist_free_one, NULL);
+  SList *next;
+
+  if (!list)
+    return;
+
+  for (SList *l = list; l; l = next)
+  {
+    next = l->next;
+    slist_free_one (l, func);
+  }
 }