R_SkinUsable sigsegv on custom map packs

Here's a regression crash 2.2.10 introduced that I wasn't able to reliably reproduce before, but could now thanks to someone on discord running into the issue:

tl;dr adding a .pk3 from the main menu produces weird SOC-related bugs that simply do not happen when the map pack is added as the game is loaded
issues like:
- sigsegvs when trying to use level select, the map command and sometimes even starting a new save

I had encountered the same crash once before during 2.2.10 prerelease from starting a new game in Chaos Domain using a custom character, but never made an issue because I wasn't able to reproduce the crash. However thanks to the new discovery I can 100% reproduce this, here are the steps:

1)Boot up the game vanilla, then add Chaos Domain via the addons menu
2)Go to the level select, pick any level and any character and it will crash

If you don't have level select unlocked here is a gamedata with it: chaosdomain.dat

RPT for 2.2.10: srb2win.RPT

I can confirm this crash does not happen in 2.2.9. Some possible culprits ordered from most to least sus: !1474 (merged) !1433 (merged) !1503 (merged)