Fix for crash when there is no gtk display

Reproduce the error by running over `ssh -x`

```
loxodo.py:223: GtkWarning: IA__gtk_clipboard_get_for_display: assertion 'display != NULL' failed
```

```
Traceback (most recent call last):
 ...
  File ".../loxodo/src/frontends/cmdline/loxodo.py", line 224, in do_show
    cb.set_text(record.passwd)
AttributeError: 'NoneType' object has no attribute 'set_text'
```