Disable io.ascii multiprocessing on Windows for now

As @embray noted in #2952, some io.ascii tests fail on Windows because there is a problem with Windows multiprocessing. This PR temporarily disables multiprocessing on Windows (and contains a minor segfault fix).
Although it occurs to me know the way this is structured wouldn't make that so easy without marking all the cases XFAIL.  If you think there's an easy way to separate out this case that would be good, but if it's too much trouble then this is fine for now.