Experimental sniper runtime fix (#4950)

# About the pull request
Fixes(?) below


![image](https://github.com/cmss13-devs/cmss13/assets/41448081/21a757b2-05e1-44b9-be0d-5c1044cbe296)

There are multiple possible causes, will look into further if this
runtime pops up again

code/modules/projectiles/guns/specialist/sniper.dm

@@ -154,7 +154,7 @@
 	target.overlays -= lockon_direction_icon
 	qdel(laser_beam)
 
-	if(!check_can_use(target, TRUE))
+	if(!check_can_use(target, TRUE) || target.is_dead())
 		return
 
 	var/obj/projectile/aimed_proj = sniper_rifle.in_chamber