[router] Fix + test routing via next/last-hop hints only

We had code in the router to support sending a payment via a single
hop across channels exclusively provided by the next-/last-hop hints.
However, in updating the fuzzer, I noted that this case not only
didn't work, but paniced in some cases.

Here, we both fix the panic, as well as write a new test which
ensures we don't break support for such routing in the future.