Fixes a minor race condition in active_displays that might happen
if a display is removed between the active_display_count and the
CGGetActiveDisplayList. In that case the returned vec would be
inconsistent and contain one (or more) spurious null displays.
Note: the returned value will always be subject to TOC/TOU,
of course, but this specific case has the returned value
containing invalid displays with id=0 that were never supposed
to be in the returned value in any possible state of the
displays; this commit fixes this case.
Marco Mastropaolo
8002ddd167