Fix corpse length

[kevingranade]

Summary

SUMMARY: None

Purpose of change

Corpse length was overlooked in #40186 resulting in #40446

Describe the solution

Fixes #40446
Overrides length in item::length() if the item is a corpse.
While I was at it, clarify the name of the function to determine a fallback item length.

Testing

Spawn various corpses and verify they have sane lengths based on their volumes.
Added a test that asserts a minimal length for all non-liquid, non-soft items, including all corpse variants.

[Erozaxx]

Hello Kevin, I did try to fix this issue before you've initiated the PR. I did not know how one can solve such conflict in GIT so I've reverted to master, applied your changes as one commit and the applied my changes as this commit [https://github.com/Erozaxx/Cataclysm-DDA/commit/30de543620cdb54693719904528a25bdfab01fff](Add possibility to define average_length for monsters in JSON)
It's first time I am contributing, so sorry if I don't know the ropes. Any comment to my changes welcome and if you think it is worth committing, I'd need some hint how to solve this branching out of your PR stuff. Another option would be to wait until it is part of the master and replicate the changes as separate PR afterwards.

[ZhilkinSerg]

Hello Kevin, I did try to fix this issue before you've initiated the PR. I did not know how one can solve such conflict in GIT so I've reverted to master, applied your changes as one commit and the applied my changes as this commit [https://github.com/Erozaxx/Cataclysm-DDA/commit/30de543620cdb54693719904528a25bdfab01fff](Add possibility to define average_length for monsters in JSON)
It's first time I am contributing, so sorry if I don't know the ropes. Any comment to my changes welcome and if you think it is worth committing, I'd need some hint how to solve this branching out of your PR stuff. Another option would be to wait until it is part of the master and replicate the changes as separate PR afterwards.

Yeah, just rebase your branch on top of current master.