copy_clear
copy_clear(uint8_t* dst, size_t dstlen, uint8_t* src, size_t srclen, size_t n);
if (copy_clear(ecs->subnet_addr, INET6_SIZE, ecs_option->opt_data + 4,
if (!copy_clear( ecs->subnet_addr, INET6_SIZE,
if (!copy_clear( ecs->subnet_addr, INET6_SIZE,