server: Fix for possible desktop variable being NULL when set_process_default_desktop is called

[Dan Kegel]

Mike Hearn wrote:
> I'm not really convinced a static
> analysis can catch things that we "know" cannot happen (eg it'd be a
> violation of the X protocol or something) and so rely on them without
> explicit checks.

To be secure, I think we have to handle X protocol violations, too.
- Dan

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