I didn't mean to change this bit, it should remain "if
(!num_threads)". I was in the middle of monkeypatching and didn't
review the diff carefully enough. But it any case, without this change
the rest of this diff is your proposed (but segfaulting) change as I
understand it.

Sorry for the proposing a sloppy alternative. (I missed one occurrence
of num_threads used in a conditional).
I think the original is still better than littering comments everywhere.