Toggle Routing Cong Cost seg faulting

[alanbu1]

Clicking an option under the "toggle routing cong cost" under the routing menu results in a segmentation fault.

[vaughnbetz]

@nedsels When you move on to graphics please test this (with and without flat routing) and see if there is still a problem. I think it is gone but am not sure.

[vaughnbetz]

@AmirhosseinPoolad : since you're playing with the graphics, can you try this option and see if there is a problem? You can try it with a successful routing (shouldn't show any congestion) and with a failed one; you can run with
--route_chan_width 100 --max_router_iterations 10 (or some other related numbers) to get the router to fail on a not-too-big circuit.

[AmirhosseinPoolad]

I think the PR I just sent fixes this, but maybe we should use at() instead of operator[] to access vectors to avoid this type of issue in the future. At least in places where performance is not as important.

[vaughnbetz]

Thanks; fixed by #2865.
We could use at() more in non-performance critical code, but running the sanitized build (or valgrind) provides equivalent checking with no runtime penalty in release build.