Fixes a race condition in mutations code (#79829)

## About The Pull Request

One of the timers has a callback to the `modify()` proc which also
doesn't check whether the mutation owner has been deleted since the
timer began, potentially resulting in a runtime.


![image](https://github.com/tgstation/tgstation/assets/13398309/1e41c48b-2620-4473-9278-702d490871cc)

## Why It's Good For The Game

Fixes bugs

## Changelog

🆑
fix: fixed a race condition with mutations
/🆑

code/datums/mutations/_mutations.dm

@@ -173,7 +173,7 @@
  * returns an instance of a power if modification was complete
  */
 /datum/mutation/human/proc/modify()
-	if(modified || !power_path || !owner)
+	if(modified || !power_path || QDELETED(owner))
 		return
 	var/datum/action/cooldown/modified_power = locate(power_path) in owner.actions
 	if(!modified_power)