siegemanager throws lua errors in the console if no siege engine is present in the fort

Probably a qerror missing around line 419

```
.../common/Dwarf Fortress/hack/scripts/gui/siegemanager.lua:419: attempt to index a nil value (local 'selected')
stack traceback:
        .../common/Dwarf Fortress/hack/scripts/gui/siegemanager.lua:419: in method 'set_selected_action'
[...]
```