middleware.CleanPath doesn't work with other routers

The rctx is not checked and could be nil if the function isn't used with a chi.Router.
It ends up in a panic.

PR: #786

Additional point: the cleaning only happens in the rctx.RoutePath is an empty string. If a function like [GetHead](https://github.com/go-chi/chi/blob/v1.5.4/middleware/get_head.go#L10) [¶](https://pkg.go.dev/github.com/go-chi/chi/middleware#GetHead) is called before it won't clean the path.