Fixed impassable furniture stopping the player from climbing

[AniDemi]

Summary

SUMMARY: Bugfixes "Fixed impassable furniture stopping the player from climbing"

Purpose of change

During gameplay I have noticed that having too many furniture around a downspout will cause it to no longer be climbable, as far I know this is unintended behaviour. This issue should also affect everything that is climbable, with a few exceptions.

Fixes #2704

Describe the solution

Made the return unable to return anything lower than 0, makes it so that the move cost won't go negative and thus stop the player from climbing.

Describe alternatives you've considered

Do nothing, it might be intended.
Cry because I accidentally removed everything I wrote.

Testing

1. Try to climb with 5 impassable furniture around, can climb
2. Try to climb with 6 impassable furniture around, can now climb unlike before.

[chaosvolt]

1. Compiled and load-tested.
2. Clambering up the evac pipe works as normal.
3. Spawned in an ever-increasing number of road barricades to fill the surrounding empty spaces, still was able to climb each time.