Fix crash when the node.name does not contain ':'

stairsplus/common.lua

@@ -50,7 +50,7 @@ stairsplus.rotate_node_aux = function(itemstack, placer, pointed_thing)
 	local under_node = minetest.get_node(under)
 	local under_prefix = under_node and name_to_category(under_node.name)
 	-- NALC Addition to avoid coloredwood/unifiedbricks crash by Sys4
-	local under_pmod = string.sub(under_node.name, 1, string.find(under_node.name, ":")-1)
+	local under_pmod = string.sub(under_node.name, 1, (string.find(under_node.name, ":") or 0)-1)
 
 	local same_cat = item_prefix == under_prefix