LP #1727920: innobackupex --slave-info report a single "m" instead of "slave info" in std output.

**Reported in Launchpad by jixiangqd last update 14-11-2017 07:56:45

Version: xtrabackup 2.4.8

I found it in

storage/innobase/xtrabackup/src/backup_mysql.cc 1388:1391
```
if (tmp_mysql_slave_position)

{ mysql_slave_position += '\n'; mysql_slave_position += *tmp_mysql_slave_position; }

```

while tmp_mysql_slave_position is typeof "char *", so that it extract "tmp_mysql_slave_position"'s first char "m" and added into output str "mysql_slave_position".
And the line 1389 is not necessary here, while earlier version's output infos inline.

this problem will fixed easy with this patch(tested):

```
if (tmp_mysql_slave_position)

{ mysql_slave_position += tmp_mysql_slave_position; }

```

Thanks you guys.