Wrong size check in memcached_last_error_message

[Martin C. Martin]

memcached_last_error_message contains the following:

      if (memc->error_messages->size == 0)
      {
        return memc->error_messages->message;
      }

      return memcached_strerror(memc, memc->error_messages->rc);

The check against "size" should be "size > 0", a size of zero means the string is empty.