Incorrect autoreplace behaviour when temporary free-wagon chains exceed the train length limit

[JGRennison]

Version of OpenTTD

1da0ba9

Expected result

Autoreplace operates correctly.

Actual result

Autoreplace operates incorrectly when a free-wagon chain is created that temporarily exceeds the train length limit by more a vehicle length, such that attempts to sell or move a vehicle in the free wagon chain fail as the remaining chain length still exceeds the limit.

This can manifest as:

• If wagon removal is off, an ever-growing wagon chain which the player is not charged for can be created by requesting an autoreplace of short wagons with long ones. (See group A in the save).
• If wagon removal is on, an assertion failure can be triggered if the depot already contains a free wagon chain of the maximum length. (See group B in the save).
  

  OpenTTD/src/autoreplace_cmd.cpp

  Line 601 in 1da0ba9

  assert(ret.Succeeded());

• Otherwise valid autoreplace operations can be blocked with a train too long error message if the depot already contains a free wagon chain of the maximum length. (See groups C vs D in the save).

Steps to reproduce

Autoreplace Testing Ltd, 1980-03-12.sav.zip

[James103]

Also for the case of group A, the "infinite free wagons" trick can and will reduce the game to <1 TPS and <1 FPS after some time has passed. The same trick can even cause the same crash as in #8295 (stack overflow in Vehicle::PreDestructor), which the free wagon chain limit was supposed to protect against.