sbus: check connection for NULL before unregister it
There seem to be code paths where the data is a added to the hash before
the connection is properly initialized, to avoid core dump during shut
down we only call dbus_conection_unregister_object_path() if there is a
connection.
Resolves:
https://pagure.io/SSSD/sssd/issue/3367
Reviewed-by: Pavel Březina <pbrezina@redhat.com>