Fix leak in ibus_get_local_machine_id()

If /var/lib/dbus/machine-id does not exist but /etc/machine-id exists,
'error' will be set but will never be freed.

BUG=http://code.google.com/p/ibus/issues/detail?id=1701

Review URL: https://codereview.appspot.com/88770043
Patch from Christophe Fergeau <cfergeau@redhat.com>.