Performance fix in dom_chars_handler: use g_strndup instead of g_strdup

Because the origin string can be extraordinarly long, you get more
benefit from this than you would imagine

libgnucash/backend/xml/sixtp-to-dom-parser.cpp

@@ -95,7 +95,7 @@ static gboolean dom_chars_handler (
 {
     if (length > 0)
     {
-        gchar* newtext = g_strdup (text);
+        gchar* newtext = g_strndup (text,length);
         xmlNodeAddContentLen ((xmlNodePtr)parent_data,
                               checked_char_cast (newtext), length);
         g_free (newtext);