vlist: define vlist_for_each_element_safe

Yet another macro wrapper around the corresponding avl_* macro.
This new macro makes it possible to iterate over vlists in ways which
may have destructive consequences without being punished by segfault.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

vlist.h

@@ -72,6 +72,9 @@ void vlist_flush_all(struct vlist_tree *tree);
 #define vlist_for_each_element(tree, element, node_member) \
 	avl_for_each_element(&(tree)->avl, element, node_member.avl)
 
+#define vlist_for_each_element_safe(tree, element, node_member, ptr) \
+	avl_for_each_element_safe(&(tree)->avl, element, node_member.avl, ptr)
+
 #define vlist_for_each_element_reverse(tree, element, node_member) \
 	avl_for_each_element_reverse(&(tree)->avl, element, node_member.avl)