Final link_idx of path_tpc is always zero

[mbbruch]

I've checked a couple networks and it appears that after the simulation is complete, the path_tpc.link_points on SpeedLimitTrainSim always concludes with index zero, rather than the correct final link.

For example, if a path should be 1 -> 2 -> 3 -> 4, the values stored on link_idx.idx will instead be 1 -> 2 -> 3 -> 0.
If the reverse direction should be 4 -> 3 -> 2 -> 1, it will instead be 4 -> 3 -> 2 -> 0.

This should reproduce on most any network, but I can confirm it does for speed_limit_train_sim_demo.py.