M39 arm brace stock disables when dropped (#4655)

# About the pull request

If you got delimbed while the brace was locked, the next person to equip
it gets it stuck on their hand
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# Explain why it's good for the game
It should fix the gun accidentally getting stuck to peoples hands

# Testing Photographs and Procedure
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# Changelog
:cl:
fix: M39 arm brace disables when dropped
/:cl:

code/modules/projectiles/gun.dm

@@ -524,6 +524,10 @@ As sniper rifles have both and weapon mods can change them as well. ..() deals w
 	if(fire_delay_group && delay_left > 0)
 		LAZYSET(user.fire_delay_next_fire, src, world.time + delay_left)
 
+	for(var/obj/item/attachable/stock/smg/collapsible/brace/current_stock in contents) //SMG armbrace folds to stop it getting stuck on people
+		if(current_stock.stock_activated)
+			current_stock.activate_attachment(src, user, turn_off = TRUE)
+
 	unwield(user)
 	set_gun_user(null)