Guncode Agony #2: Fixes harddels caused by certain projectiles (collosus bolts in CI) (#87534)

## About The Pull Request

Closes #87349
This may seem concerning but projectiles always assume that their firer
and original can be null at any step, so this is completely safe.

## Changelog
🆑
fix: Fixed harddels caused by certain projectiles
/🆑

code/modules/projectiles/projectile.dm

@@ -764,7 +764,10 @@
 	if(fired_from)
 		SEND_SIGNAL(fired_from, COMSIG_PROJECTILE_BEFORE_FIRE, src, original)
 	if(firer)
+		RegisterSignal(firer, COMSIG_QDELETING, PROC_REF(firer_deleted))
 		SEND_SIGNAL(firer, COMSIG_PROJECTILE_FIRER_BEFORE_FIRE, src, fired_from, original)
+	if (original)
+		RegisterSignal(original, COMSIG_QDELETING, PROC_REF(original_deleted))
 	if(!log_override && firer && original && !do_not_log)
 		log_combat(firer, original, "fired at", src, "from [get_area_name(src, TRUE)]")
 			//note: mecha projectile logging is handled in /obj/item/mecha_parts/mecha_equipment/weapon/action(). try to keep these messages roughly the sameish just for consistency's sake.
@@ -814,6 +817,14 @@
 		store_hitscan_collision(point_cache)
 	return TRUE
 
+/obj/projectile/proc/firer_deleted(datum/source)
+	SIGNAL_HANDLER
+	firer = null
+
+/obj/projectile/proc/original_deleted(datum/source)
+	SIGNAL_HANDLER
+	original = null
+
 /// Same as set_angle, but the reflection continues from the center of the object that reflects it instead of the side
 /obj/projectile/proc/set_angle_centered(new_angle)
 	if(!nondirectional_sprite)
@@ -1064,6 +1075,8 @@
 		finalize_hitscan_and_generate_tracers()
 	STOP_PROCESSING(SSprojectiles, src)
 	cleanup_beam_segments()
+	firer = null
+	original = null
 	if(trajectory)
 		QDEL_NULL(trajectory)
 	return ..()