The coronavirus scourge is a prevailing theme at this year’s United Nations General Assembly, forcing the gathering to be conducted largely online. But the pandemic is also fueling another crisis preoccupying the organization and humanitarian groups: the strong prospect of famine in some of the world’s most destitute places. Nowhere is famine more likely than in Yemen, the Arab world’s poorest country, ravaged by war for nearly six years between the Houthi rebels and a Saudi-led military coalition defending a weak government that exerts little or no control over most Yemeni territory.