Avoid using length/1

Author: kipcole9

lib/vix/vips/image.ex

@@ -1548,7 +1548,7 @@ defmodule Vix.Vips.Image do
         !is_list(list) ->
           {:error, "argument is not a list"}
 
-        length(list) > 0 && is_list(hd(list)) ->
+        non_empty_list?(list) && is_list(hd(list)) ->
           height = length(list)
           width = length(hd(list))
 
@@ -1574,6 +1574,9 @@ defmodule Vix.Vips.Image do
     end
   end
 
+  defp non_empty_list?([]), do: false
+  defp non_empty_list?(_list), do: true
+
   defp validate_list_dimension(width, height, list) do
     if length(list) == width * height do
       :ok