Fix crash when leaves don't have a registered orphan but try to rot

mods/ITEMS/mcl_core/nodes_trees.lua

@@ -35,7 +35,14 @@ function mcl_core.update_leaves(pos, oldnode)
 				-- manually placed leaf nodes have param2
 				-- set and will never decay automatically
 				if lnode.param2 == 0 then
-					minetest.swap_node(lpos, {name = lnode.name .. "_orphan"})
+					local orphan_name = lnode.name .. "_orphan"
+					local def = minetest.registered_nodes[orphan_name]
+					if def then
+						--minetest.log("Registered: ".. orphan_name)
+						minetest.swap_node(lpos, {name = orphan_name})
+					else
+						--minetest.log("Not registered: ".. orphan_name)
+					end
 				end
 			end
 		end