An Israeli airstrike last month demolished the top floor of a guest house in Gaza where World Food Program international staff were staying, the U.N. agency’s director said Thursday, calling the situation “impossibly dangerous” for aid workers trying to feed the Palestinian population. The previously undisclosed incident occurred 31 August in the Nuseirat refugee camp, just days after WFP temporarily stopped aid deliveries to northern Gaza and halted staff movements when its team came under fire near an Israel checkpoint. “It was always dangerous before. It’s become impossibly dangerous now,’’…