parser: Remove unnecessary dict checks when freeing strings The following strings are never allocated from a dict: - xmlParserCtxt.version - xmlParserCtxt.encoding - xmlParserCtxt.extSubURI - xmlParserCtxt.extSubSystem - xmlDoc.version - xmlDoc.encoding - xmlDoc.URL - xmlDTD.ExternalID - xmlDTD.SystemID - xmlID.value Also make the struct members point to non-const chars to avoid casts when freeing.