Fixes runtime spam caused by altclicking tiles

Fixes every unarmed altclick on turfs runtiming because nothing bothered to check whether the person was actually holding anything to engrave with.

code/game/turfs/turf.dm

@@ -310,7 +310,7 @@
 
 /turf/proc/try_graffiti(var/mob/vandal, var/obj/item/tool)
 
-	if(!tool.sharp || !can_engrave())
+	if(!tool || !tool.sharp || !can_engrave())
 		return FALSE
 
 	if(jobban_isbanned(vandal, "Graffiti"))
@@ -407,4 +407,4 @@
 	return
 // We were the the B-side in a turf translation
 /turf/proc/post_translate_B(var/turf/A)
-	return
+	return