lib/a_star: avoid bad autocast

Seriously, this one case might be a strong argument against autocasts

~~~
frontier_node = frontier_node.best_source.as(not null)
~~~

is auto-casted as

~~~
frontier_node = frontier_node.best_source.as(not null).as(N)
~~~

bun since the N is `nullable Node`, the not-null information was lost in the
cast.

Signed-off-by: Jean Privat <jean@pryen.org>