stairs: refactor wool registration, fixes wool:yellow stairs entry

stairs.lua

-for _,name in ipairs({"red", "green", "white", "black", "blue", "brown", "cyan",
-                    "grey", "dark_green", "dark_grey", "magenta", "orange", "pink",
-                    "red", "violet", "yellow"}) do
+local name = {"red", "green", "white", "black", "blue", "brown", "cyan",
+            "grey", "dark_green", "dark_grey", "magenta", "orange", "pink",
+            "red", "violet", "yellow"}
+for i = 1, 16 do
+    local name = name[i]
     local nodename = "wool:"..name
     local tmp = minetest.registered_nodes[nodename]
     if not tmp then