fix: update node GC lifecycle to avoid leaking fragment nodes

Upstream GNOME/libxml2@7e462425 updated `xmlAddChild` to do nothing if
the subject node's `->next` or `->prev` are non-NULL.

For fragments that are parsed in-node-context, the nodes are parsed as
siblings of each other, meaning that unless we explicitly set `next`
and `prev` to NULL, the node will not actually be inserted into the
tree by `xml_document.c:dealloc_node_i2` and so `xmlFreeDoc` will not
be able to free those nodes, resulting in a leak.

Author: flavorjones

CHANGELOG.md

@@ -16,6 +16,7 @@ Nokogiri follows [Semantic Versioning](https://semver.org/), please see the [REA
 
 * [CRuby] libgumbo (the HTML5 parser) treats reaching max-depth as EOF. This addresses a class of issues when the parser is interrupted in this way. [#3121] @stevecheckoway
 * `Node#clone`, `NodeSet#clone`, and `*::Document#clone` all properly copy the metaclass of the original as expected. Previously, `#clone` had been aliased to `#dup` for these classes (since v1.3.0 in 2009). [#316, #3117] @flavorjones
+* [CRuby] Update node GC lifecycle to avoid a potential memory leak with fragments in libxml 2.13.0 caused by changes in `xmlAddChild`. [#3156] @flavorjones
 
 
 ### Changed

ext/nokogiri/xml_document.c

@@ -17,6 +17,8 @@ dealloc_node_i2(xmlNodePtr key, xmlNodePtr node, xmlDocPtr doc)
       break;
     default:
       if (node->parent == NULL) {
+        node->next = NULL;
+        node->prev = NULL;
         xmlAddChild((xmlNodePtr)doc, node);
       }
   }