svscan exits immediately if no service directory found on startup

```console
$ cd daemontools-encore-1.11
$ sh makemake
...
$ make
...
$ mkdir empty-dir
$ time ./svscan empty-dir
./svscan empty-dir  0.00s user 0.00s system 0% cpu 0.001 total
```

daemontools 0.76 and daemontools-encore-1.03 are no problem.