Fixes 2 gun runtime sources (#4817)

# About the pull request

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This fixes two issues happening on live game:

* The XM88 does not check for successful wielding before registering
signals, presumably causing overriding errors in logs.
* M56D/M2C HMGs do not correctly reset the autofire component when the
operator is removed. This causes it to try to keep firing without an
operator which generates runtimes (presumably in burst fire mode). In
addition this may solve other issues such as the fabled "can use HMG
while dead" one, but I can't confirm it.

# Explain why it's good for the game
Readable logs and less clunky interactions down the line

# Changelog
:cl:
fix: M56D/M2C should now properly stop firing when they stop being used.
/:cl:

code/datums/components/autofire/autofire.dm

@@ -123,19 +123,19 @@
 		if(GUN_FIREMODE_BURSTFIRE)
 			shots_fired++
 			if(shots_fired == burst_shots_to_fire)
-				callback_bursting.Invoke(FALSE)
-				callback_display_ammo.Invoke()
+				callback_bursting?.Invoke(FALSE)
+				callback_display_ammo?.Invoke()
 				bursting = FALSE
 				stop_firing()
 				if(have_to_reset_at_burst_end)//We failed to reset because we were bursting, we do it now
-					callback_reset_fire.Invoke()
+					callback_reset_fire?.Invoke()
 					have_to_reset_at_burst_end = FALSE
 				return
-			callback_bursting.Invoke(TRUE)
+			callback_bursting?.Invoke(TRUE)
 			bursting = TRUE
 			next_fire = world.time + burstfire_shot_delay
 		if(GUN_FIREMODE_AUTOMATIC)
-			callback_set_firing.Invoke(TRUE)
+			callback_set_firing?.Invoke(TRUE)
 			next_fire = world.time + (auto_fire_shot_delay * automatic_delay_mult)
 		if(GUN_FIREMODE_SEMIAUTO)
 			return

code/modules/cm_marines/smartgun_mount.dm

@@ -475,8 +475,6 @@
 	var/autofire_slow_mult = 1
 	/// If the gun is currently burst firing
 	VAR_PROTECTED/burst_firing = FALSE
-	/// If the gun is currently auto firing
-	VAR_PROTECTED/auto_firing = FALSE
 	/// If the gun should display its ammo count
 	var/display_ammo = TRUE
 	/// How many degrees in each direction the gun should be able to fire
@@ -514,13 +512,14 @@
 	ammo = GLOB.ammo_list[ammo] //dunno how this works but just sliding this in from sentry-code.
 	burst_scatter_mult = SCATTER_AMOUNT_TIER_7
 	update_icon()
-	AddComponent(/datum/component/automatedfire/autofire, fire_delay, burst_fire_delay, burst_amount, gun_firemode, autofire_slow_mult, CALLBACK(src, PROC_REF(set_burst_firing)), CALLBACK(src, PROC_REF(reset_fire)), CALLBACK(src, PROC_REF(try_fire)), CALLBACK(src, PROC_REF(display_ammo)), CALLBACK(src, PROC_REF(set_auto_firing)))
+	AddComponent(/datum/component/automatedfire/autofire, fire_delay, burst_fire_delay, burst_amount, gun_firemode, autofire_slow_mult, CALLBACK(src, PROC_REF(set_burst_firing)), CALLBACK(src, PROC_REF(reset_fire)), CALLBACK(src, PROC_REF(try_fire)), CALLBACK(src, PROC_REF(display_ammo)))
 
-/obj/structure/machinery/m56d_hmg/Destroy() //Make sure we pick up our trash.
-	if(operator)
-		operator.unset_interaction()
+/obj/structure/machinery/m56d_hmg/Destroy(force) //Make sure we pick up our trash.
+	operator?.unset_interaction()
+	operator = null
+	QDEL_NULL(in_chamber)
 	STOP_PROCESSING(SSobj, src)
-	. = ..()
+	return ..()
 
 /obj/structure/machinery/m56d_hmg/get_examine_text(mob/user) //Let us see how much ammo we got in this thing.
 	. = ..()
@@ -751,6 +750,8 @@
 	update_icon() //final safeguard.
 
 /obj/structure/machinery/m56d_hmg/proc/try_fire()
+	if(!operator)
+		return
 	if(!rounds)
 		to_chat(operator, SPAN_WARNING("<b>*click*</b>"))
 		playsound(src, 'sound/weapons/gun_empty.ogg', 25, 1, 5)
@@ -855,6 +856,7 @@
 
 /obj/structure/machinery/m56d_hmg/on_unset_interaction(mob/user)
 	flags_atom &= ~RELAY_CLICK
+	SEND_SIGNAL(src, COMSIG_GUN_INTERRUPT_FIRE)
 	user.status_flags &= ~IMMOBILE_ACTION
 	user.visible_message(SPAN_NOTICE("[user] lets go of \the [src]."),SPAN_NOTICE("You let go of \the [src], letting the gun rest."))
 	user.unfreeze()
@@ -947,7 +949,6 @@
 /// Clean up the target, shots fired, and other things related to when you stop firing
 /obj/structure/machinery/m56d_hmg/proc/reset_fire()
 	set_target(null)
-	set_auto_firing(FALSE)
 	shots_fired = 0
 
 ///Set the target and take care of hard delete
@@ -960,10 +961,6 @@
 	if(target)
 		RegisterSignal(target, COMSIG_PARENT_QDELETING, PROC_REF(clean_target))
 
-/// Setter for auto_firing
-/obj/structure/machinery/m56d_hmg/proc/set_auto_firing(auto = FALSE)
-	auto_firing = auto
-
 /// Print how much ammo is left to chat
 /obj/structure/machinery/m56d_hmg/proc/display_ammo()
 	if(!operator)

code/modules/projectiles/gun.dm

@@ -1963,6 +1963,7 @@ not all weapons use normal magazines etc. load_into_chamber() itself is designed
 
 /// Setter proc for fa_firing
 /obj/item/weapon/gun/proc/set_auto_firing(auto = FALSE)
+	SIGNAL_HANDLER
 	fa_firing = auto
 
 /// Getter for gun_user

code/modules/projectiles/guns/lever_action.dm

@@ -405,9 +405,9 @@ their unique feature is that a direct hit will buff your damage and firerate
 
 /obj/item/weapon/gun/lever_action/xm88/wield(mob/user)
 	. = ..()
-
-	RegisterSignal(src, COMSIG_ITEM_ZOOM, PROC_REF(scope_on))
-	RegisterSignal(src, COMSIG_ITEM_UNZOOM, PROC_REF(scope_off))
+	if(.)
+		RegisterSignal(src, COMSIG_ITEM_ZOOM, PROC_REF(scope_on))
+		RegisterSignal(src, COMSIG_ITEM_UNZOOM, PROC_REF(scope_off))
 
 /obj/item/weapon/gun/lever_action/xm88/proc/scope_on(atom/source, mob/current_user)
 	SIGNAL_HANDLER