Panic at the beginning of zoomed in crosswalk turns sometimes

Pulling this up from the comments of #816. A crash can occur if a pedestrian starts a contraflow turn just right, causing `PolyLine::must_dist_along` to be called with a certain value.

Here is abstreet/data/system/us/nyc/maps/fordham.bin
[fordham.bin.zip](https://github.com/a-b-street/abstreet/files/7748529/fordham.bin.zip)

For me it crashes at 00:02:00.0

You will need to look at somewhere in the upper area of the map, e.g. intersection 82.

_Originally posted by @mdejean in https://github.com/a-b-street/abstreet/issues/816#issuecomment-998351470_