Nokogiri: Possible Use-After-Free when setting `Document#root=` to an invalid node type

Summary Nokogiri::XML::Documentroot= validated only that the new root was a Nokogiri::XML::Node, allowing a DTD node to be set as the document root. The result is a heap use-after-free during garbage collection or finalization, leading to an invalid memory read or potentially a segfault. Nokogiri...