Handle config file with service without name

If part of config file looks like:

service {
    ver: 1
}

corosync will read unitialized memory probably resulting in fail.
Solution is to properly check result of object_key_get.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>