Segfault in example/router

For example:

```
auto tmp_r = new urls::router<handler>;
urls::router<handler> r = *tmp_r; // copy impl_ pointer from router_base
delete tmp_r; // delete impl_ pointer

r.insert("/", [&](connection& c, urls::matches const&) { // segfault
    c.string_reply("Hello!");
});
```