innobackupex does not handle non-slave mysql with safe-slave-backup option

When using --safe-slave-backup option innobackupex will correctly detect whether or not the host is a slave in the wait_for_safe_slave() function and not attempt to STOP/START slave, however, the script will still attempt to restart the slave later on. If the host is not a slave, this will cause innobackupex to exit with non-zero exit code, despite the issue being benign.