Reloading spent casings in the revolver doesn't duplicate "This one is spent" message

Fixed #2434.

code/modules/projectiles/guns/projectile.dm

@@ -37,8 +37,8 @@
 	if(isnull(AC) || !istype(AC))
 		return 0
 	AC.loc = get_turf(src) //Eject casing onto ground.
-	AC.desc += " This one is spent."	//descriptions are magic
 	if(AC.BB)
+		AC.desc += " This one is spent."	//descriptions are magic - only when there's a projectile in the casing
 		in_chamber = AC.BB //Load projectile into chamber.
 		AC.BB.loc = src //Set projectile loc to gun.
 		return 1