[sixtp-dom-parsers.cpp] introduce dom_node_to_text for solo text node

allocation-free
returns const char* or nullptr

libgnucash/backend/xml/sixtp-dom-parsers.cpp

@@ -35,6 +35,15 @@
 
 static QofLogModule log_module = GNC_MOD_IO;
 
+const char*
+dom_node_to_text (xmlNodePtr node) noexcept
+{
+    if (node && node->children && node->children->type == XML_TEXT_NODE
+        && !node->children->next)
+        return reinterpret_cast<const char*>(node->children->content);
+    return nullptr;
+}
+
 std::optional<GncGUID>
 dom_tree_to_guid (xmlNodePtr node)
 {

libgnucash/backend/xml/sixtp-dom-parsers.h

@@ -45,6 +45,7 @@ gboolean dom_tree_valid_time64 (time64 ts, const xmlChar* name);
 GDate* dom_tree_to_gdate (xmlNodePtr node);
 gnc_numeric dom_tree_to_gnc_numeric (xmlNodePtr node);
 gchar* dom_tree_to_text (xmlNodePtr tree);
+const char* dom_node_to_text (xmlNodePtr node) noexcept;
 gboolean string_to_binary (const gchar* str,  void** v, guint64* data_len);
 gboolean dom_tree_create_instance_slots (xmlNodePtr node, QofInstance* inst);