Sling can become unusable due to 'ghost' item attached

### Testmerges

from round after. #10999: 'Fix of various plane related issues' by Blundir at commit f85a7153c4 #11389: 'Yautja Leap' by realforest2001 at commit 0fd7966dd6 #11214: 'Proof of concept movielike pred invisibility' by Blundir at commit d8fc73ab73 #9404: 'Guns can now drop bullet casings' by Nihisohel at commit 448bf01707 #11459: 'admin ranks from api' by hry-gh at commit 89509ee4eb #10823: 'Abyssal Station - (Lowpop) Ground Map' by Steelpoint at commit 846d0594f2 #11582: 'Fixes FTLs being unable to pick their Essential Fireteam kit' by Nihisohel at commit f0f9043d52 #9774: 'Adds a minesweeper game to a bunch of interactions.' by kiVts at commit ea0ae40948

### Round ID

32017

### Description of the bug

1. have a defib attached to a sling
2. get hit by mortar, defib is no longer in hand or on sling
3. sling won't accept a new item as it thinks it still has the defib

i was recording during the round and ill post a clip if you need it

### What's the difference with what should have happened?

should accept a new item if the attached item is destroyed or gone for whatever reason

### How do we reproduce this bug?

1. have a defib attached to a sling
2. get hit by mortar, defib is no longer in hand or on sling
3. sling won't accept a new item as it thinks it still has the defib

### Issue Bingo

- [ ] Issue could be reproduced at least once
- [x] Issue happened in a recent (less than 7 days ago) round
- [x] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)