[libxml2] Fix freeing of nested documents

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 0815302dee2b78139832c2080348086a0564836b
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Fri Dec 6 12:27:29 2019 +0100

    Fix freeing of nested documents
    
    Apparently, some libxslt RVTs can contain nested document nodes, see
    issue #132. I'm not sure how this happens exactly but it can cause a
    segfault in xmlFreeNodeList after the changes in commit 0762c9b6.
    
    Make sure not to touch the (nonexistent) `content` member of xmlDocs.

 tree.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/tree.c b/tree.c
index 070670f1..0d7fc98c 100644
--- a/tree.c
+++ b/tree.c
@@ -3708,6 +3708,11 @@ xmlFreeNodeList(xmlNodePtr cur) {
                (cur->type != XML_XINCLUDE_START) &&
                (cur->type != XML_XINCLUDE_END) &&
                (cur->type != XML_ENTITY_REF_NODE) &&
+               (cur->type != XML_DOCUMENT_NODE) &&
+#ifdef LIBXML_DOCB_ENABLED
+               (cur->type != XML_DOCB_DOCUMENT_NODE) &&
+#endif
+               (cur->type != XML_HTML_DOCUMENT_NODE) &&
                (cur->content != (xmlChar *) &(cur->properties))) {
                DICT_FREE(cur->content)
            }