[11.6] Crash when assigning schedule

[simgamer]

Steps to reproduce bug:
- create new line in "Line Management"
- leave "Schedule" window open.
- Click on a vehicle that is somewhere on the road and assign it the new schedule.
- Close the "Schedule" window of the vehicle
- Game must not be in paused mode now
- Close the "Schedule" window of the line

Result:
FATAL ERROR: minivec_tpl<T>::[] index out of bounds: 255 not in 0..-1

[jamespetts]

Thank you for the report - found and fixed on the 11.x branch.