this fixes galera.galera_sst_mariabackup_table_options

Note that `man snprintf` says

  The functions snprintf() and vsnprintf() do not write more
  than size bytes (including the terminating null byte
  ('\0')). If the output was truncated due to this limit, then
  the return value is the number of characters (excluding the
  terminating null byte) which would have been written to the
  final string if enough space had been available.