catch links with costs <= 0 earlier in program flow

[kainagel]

As we know, this leads to problems with routing. Unfortunately, this is non-deterministic, since the routers do not always explore all links systematically, and so in some run one may encounter the problem and in some other run not. Also, switching from one type of router (e.g. Dijkstra) to some other (e.g. landmarks) also changes the links that are explored.

There were also problems with multithreaded routing: matsim-org/matsim-code-examples#258 . Removing the above problem as source of errors would allow to concentrate on the multithreading issue.