fix setting invalid y in pbuf in ceiling/floor placement

src/main.lua

@@ -409,7 +409,7 @@ local function printdes(buf, dimensions)
 
             cx,cz,direction = moveto(target[1],target[2],cx,cz,direction)
             place("down")
-            pbuf[posasstring(cx,cy,cz)] = true
+            pbuf[posasstring(cx,0,cz)] = true
         end
         -- return to standard pos but +1 to y
         cx,cz,direction = moveto(1,1,cx,cz,direction)