Guard against null dereference when no workers are running

[PhirePhly]

I usually assume 0 is an int data type. I'd rather use NULL for checking pointer equivalency.

[fkautz]

I've updated the PR to use NULL instead of 0

[hessu]

Hmm, it logs a LOG_INFO message but does not actually guard or return from the function, was that intentional?

If workers_running > 0 && worker_threads == NULL, it's quite a critical corruption at that point, might make sense to log at LOG_CRIT level and abort() out.

[fkautz]

@hessu I've updated the patch to log at CRIT level and abort as recommended.

[hessu]

Thank you, merged!