identity: plug leak in identity service

When the identity service first starts up, it gets a list
of preexisting kerberos principals and then exports them
over the bus.

The object path used for the exported objects is getting leaked.
This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=739590
kerberos: Zero initialize operation structure

We need zero initialize the structure so we don't end up
freeing unused members when cleaning up the operation.

https://bugzilla.gnome.org/show_bug.cgi?id=739593