Fixes RPD/disposals exploits

Also removes a bit of copy-paste and speeds up a helper slightly.

Fixes #13473
Fixes #14654

code/game/objects/items/weapons/RPD.dm

@@ -529,7 +529,7 @@ var/global/list/RPD_recipes=list(
 
 	if(istype(A,/area/shuttle)||istype(A,/turf/space/transit))
 		return 0
-
+	
 	//So that changing the menu settings doesn't affect the pipes already being built.
 	var/queued_p_type = p_type
 	var/queued_p_dir = p_dir
@@ -592,7 +592,7 @@ var/global/list/RPD_recipes=list(
 			return 0
 
 		if(DISPOSALS_MODE)
-			if(!(istype(A, /turf)))
+			if(!isturf(A) || is_anchored_dense_turf(A))
 				user << "<span class='warning'>The [src]'s error light flickers!</span>"
 				return 0
 			user << "<span class='notice'>You start building pipes...</span>"