fix memsize_node when called on xmlAttrs

The `properties` field of an `xmlNode` element points to an `xmlAttr`.
The first few fields of `xmlAttr` are in common with `xmlNode`, but not
the `properties` field which doesn't exist in an `xmlAttr`.

The `memsize_node` function was passing an `xmlAttr` to a
recursive call and then trying to do the same with the properties of
that.

This led to type confusion and subsequent crashes.

Fixes: #2923
(cherry picked from commit 81762fa)

ext/nokogiri/xml_document.c

@@ -103,8 +103,11 @@ memsize_node(const xmlNodePtr node)
   size_t memsize = 0;
 
   memsize += xmlStrlen(node->name);
-  for (child = (xmlNodePtr)node->properties; child; child = child->next) {
-    memsize += sizeof(xmlAttr) + memsize_node(child);
+
+  if (node->type == XML_ELEMENT_NODE) {
+    for (child = (xmlNodePtr)node->properties; child; child = child->next) {
+      memsize += sizeof(xmlAttr) + memsize_node(child);
+    }
   }
   if (node->type == XML_TEXT_NODE) {
     memsize += xmlStrlen(node->content);