If there are any literals available in the router, then any access to a path with
multiple segments and its first segment an object prototype property lead to a
TypeError.

This fixes these TypeErrors by checking whether the property is the literal list's own.

There is a similar code path for the `this._fulls` but the path always contains a leading slash,
thus is not vulnerable to such error.

```
Debug: internal, implementation, error
    TypeError: match.lookup is not a function
    at Object.internals.deeper (/some-project/node_modules/hapi/node_modules/call/lib/segment.js:240:28)
    at internals.Segment.lookup (/some-project/node_modules/hapi/node_modules/call/lib/segment.js:177:36)
    at internals.Router._lookup (/some-project/node_modules/hapi/node_modules/call/lib/index.js:113:28)
    at internals.Router.route (/some-project/node_modules/hapi/node_modules/call/lib/index.js:94:22)
    at internals.Request._lifecycle (/some-project/node_modules/hapi/lib/request.js:343:41)
    at internals.Request._execute (/some-project/node_modules/hapi/lib/request.js:312:21)
    at Domain.<anonymous> (/some-project/node_modules/hapi/lib/connection.js:253:25)
    at Domain.run (domain.js:191:14)
    at internals.Protect.enter (/some-project/node_modules/hapi/lib/protect.js:84:17)
    at Server.internals.Connection._dispatch (/some-project/node_modules/hapi/lib/connection.js:251:30)
```