fix: hold on reference to prevent early GC

Author: Tieske

xmlua/document.lua

@@ -224,7 +224,9 @@ do  -- C14N methods
     set.nodeMax = #nodes
     set.nodeTab = nodeTab
 
-    return set
+    return ffi.gc(set, function(ptr)
+      nodeTab = nil -- release references, so they can be GC'ed
+    end)
   end