It’s the breast defense — a young woman can’t be held responsible in a fatal crash because she faced a roadway emergency after a back-seat passenger unfastened her bikini top, exposing her assets. That was the ruling handed down last week by a Brooklyn appeals court, providing the latest twist in the bizarre and tragic case that unfolded after Manhattanite Brittany Lahm, 24, crashed on the Thruway in July 2008.