Guards against runtiming in the middle of the client Destroy() proc (#92686)

## About The Pull Request

Tin.

Runtiming in this spot is really not good and may have potentially
caused a crash for one of our rounds.

Not sure how it happened exactly, but it costs nothing to be safe here.

<img width="696" height="38" alt="image"
src="https://github.com/user-attachments/assets/c56c5c18-8ae0-428f-a407-b2e3de79d87a"
/>

## Why It's Good For The Game

Let clients finish their destruction

## Changelog

Not player-facing

code/modules/client/client_procs.dm

@@ -596,7 +596,10 @@ GLOBAL_LIST_INIT(blacklisted_builds, list(
 
 	GLOB.clients -= src
 	GLOB.directory -= ckey
-	persistent_client.set_client(null)
+	if(persistent_client)
+		persistent_client.set_client(null)
+	else
+		stack_trace("A client was Del()'d without a persistent_client! This should not be happening.")
 
 	log_access("Logout: [key_name(src)]")
 	GLOB.ahelp_tickets.ClientLogout(src)