bug in HA_TOPTION_SYSVAR of the enum type - enum string was generated incorrectly

sql/create_options.cc

@@ -395,6 +395,7 @@ static bool resolve_sysvars(handlerton *hton, ha_create_table_option *rules)
 
         char buf[256];
         String str(buf, sizeof(buf), system_charset_info);
+        str.length(0);
         for (const char **s= optp.typelib->type_names; *s; s++)
         {
           if (str.append(*s) || str.append(','))