DS doorgun no longer drops the gun when destroyed (#5211)

# About the pull request

this bug has been around for a looong time, someone might be tempted to
abuse it with the MG changes

# Explain why it's good for the game

moveable m56d with 1500 rounds that can shoot over cover is not intended


# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog

:cl:
fix: DS doorgun no longer drops the gun when destroyed
/:cl:

---------

Co-authored-by: vincibrv <[email protected]>
Co-authored-by: Drathek <[email protected]>

code/modules/cm_marines/dropship_equipment.dm

@@ -300,6 +300,10 @@
 		deployed_mg = new(src)
 		deployed_mg.deployment_system = src
 
+/obj/structure/dropship_equipment/mg_holder/Destroy()
+	QDEL_NULL(deployed_mg)
+	. = ..()
+
 /obj/structure/dropship_equipment/mg_holder/get_examine_text(mob/user)
 	. = ..()
 	if(!deployed_mg)
@@ -360,7 +364,7 @@
 			deployed_mg.forceMove(src)
 			deployed_mg.setDir(dir)
 		else
-			icon_state = "mg_system_destroyed"
+			icon_state = "sentry_system_destroyed"
 
 /obj/structure/dropship_equipment/mg_holder/proc/deploy_mg(mob/user)
 	if(deployed_mg)

code/modules/cm_marines/smartgun_mount.dm

@@ -1082,6 +1082,18 @@
 	zoom = 1
 	ammo = /datum/ammo/bullet/machinegun/doorgun
 
+/obj/structure/machinery/m56d_hmg/mg_turret/update_health(amount) //Negative values restores health.
+	health -= amount
+	if(health <= 0)
+		playsound(src.loc, 'sound/items/Welder2.ogg', 25, 1)
+		qdel(src)
+		return
+
+	if(health > health_max)
+		health = health_max
+	update_damage_state()
+	update_icon()
+
 /obj/structure/machinery/m56d_hmg/mg_turret/dropship
 	name = "\improper scoped M56D heavy machine gun"
 	desc = "A scoped M56D heavy machine gun mounted behind a metal shield. Drag its sprite onto yourself to man it. Ctrl-click it to toggle burst fire."