NEW YORK — The Woodstock 50 festival in Watkins Glen is back on after a court rebuffed an ex-investor's effort to cancel the anniversary extravaganza — but organizers will have to do without some $18 million, at least for now.Manhattan judge Barry Ostrager ruled Wednesday that the festival's former chief backer, Amplifi Live, couldn't singlehandedly call off the August show but also doesn't have to put the $18 million back into it.