Fixes deleted mobs not unbuckling (#4717)

# About the pull request

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This makes mobs unbuckle on deletion.

Why this matters:
 - Leftover references as buckled_mob on the buckled object
- The object cannot be used by someone else, since someone is still
buckled to it
- In most extreme case eg. shuttle chairs, the shuttle will pull the
deleted mob out of nullspace and back to chair when traveling

# Explain why it's good for the game
Less bugs and less confused admins having to re-spawn an entire tank
over it

# Testing Photographs and Procedure
Tested buckling/deleting without errors

code/game/objects/objs.dm

@@ -226,9 +226,14 @@
 /obj/proc/afterbuckle(mob/M as mob) // Called after somebody buckled / unbuckled
 	handle_rotation()
 	SEND_SIGNAL(src, COSMIG_OBJ_AFTER_BUCKLE, buckled_mob)
+	if(!buckled_mob)
+		UnregisterSignal(M, COMSIG_PARENT_QDELETING)
+	else
+		RegisterSignal(buckled_mob, COMSIG_PARENT_QDELETING, PROC_REF(unbuckle))
 	return buckled_mob
 
 /obj/proc/unbuckle()
+	SIGNAL_HANDLER
 	if(buckled_mob && buckled_mob.buckled == src)
 		buckled_mob.buckled = null
 		buckled_mob.anchored = initial(buckled_mob.anchored)