This fixes a race which can be hit within a few thousand iterations of:

loop do |i|
  IO.popen4('echo', 'hi') { |_, _, out, _| out.read }
end

I believe the descriptor is occasionally closed between the test of isOpen() and the close(), triggering the exception.

I might feel better if we did both, actually, to avoid unnecessarily raising an error that gets ignored. The failed race would then be the only case that raises.

Thanks!