Fix split vehicles always using the incorrect pivot point

[Kamayana]

Summary

Bugfixes "Fix split vehicles always using the incorrect pivot point"

Purpose of change

In vehicle::split_vehicles, every new vehicle created is supposed to get a new position and pivot based on the parts they have after the split. However, when the function checks to make sure the new pivot is a valid 0,0 part, it actually mistakenly assigns the pivot to the part index of the original vehicle, AKA the other half of the split. This makes both resulting halves of the vehicle end up with the same position and pivot points, which can cause issues with anything that relies on a vehicle's position, like vehicle::get_remote_part.

Describe the solution

Set split_part0 to the value inside the split_parts array, not to the index.

Describe alternatives you've considered

Testing

In debug mode, watch the position values of the split vehicles during the relevant functions. See that after splitting the vehicles up they both incorrectly have the same position in the end. Apply this fix, do it again, and see that both splits correctly end up with different positions.

Additional context

This is a cherry-picked commit from my larger power grid usability PR, since as a bug fix it deserves its own PR.

[Procyonae]

Does this want backporting to 0.H?

[Kamayana]

Does this want backporting to 0.H?

It should, yeah. #73056 too, if possible. I was waiting to see if anyone submitted any issues with them but I haven't seen anything so I think it should be okay.