Fixes #5941 - Handle nil comparison object in DHCP::Record.valid?

As tested in IRC:

```
[15:00:35] <gwmngilfen> wojtasfs: try applying this patch http://pastie.org/9223201
[15:00:42] <gwmngilfen> then restart foreman and retry your script
[15:02:02] <wojtasfs> trying
[15:02:42] <wojtasfs> jupi! it's working, great thanks ;)
```