Add assert before we wander off into the weeds.

Visiting slashdot we get a document write with a node that has no
html content user data.

javascript/duktape/Document.bnd

@@ -31,6 +31,7 @@ method Document::write()
 	err = dom_node_get_user_data(priv->parent.node,
 				     corestring_dom___ns_key_html_content_data,
 				     &htmlc);
+	assert(htmlc != NULL);
 	if (err == DOM_NO_ERR && htmlc->parser != NULL) {
 		dom_hubbub_parser_insert_chunk(htmlc->parser, (uint8_t *)text, text_len);
 	}