[Bug]: Train head autoreplace operations leak a unitnumber in the case where there are 1 or more wagons

[JGRennison]

Version of OpenTTD

master (presumably since #12165)

Expected result

No unit number leaks

Actual result

Autoreplacing a train head engine with one or attached wagons leaks a unit number

Steps to reproduce

Buy a locomotive (train 1)
Buy a wagon
Add an auto-replace from the locomotive engine to something else
Press the auto-replace button in the depot window
Buy another locomotive (train 3)

[James103]

If you sell all trains after performing the above steps to reproduce, then buy two more trains, does the second train have unitnumber 2 (Train 2) or 3 (Train 3)?

Edit: I was able to reproduce on b8b0181. Furthermore, the unitnumber becomes unavailable for the company until the game is saved and loaded.