A sell-off put Apple at risk of losing its status as the world’s most valuable company. Thursday, it was worth $423 billion, just 1.6 percent more than Exxon Mobil. The drop was set off by Apple’s earnings Wednesday, which suggested its long growth spurt is slowing. Profit was flat over the year before. Of even more concern to investors: Forecast sales growth for the current quarter is around 7 percent vs. a year ago — far from the 50-percent-plus rate it has often hit in recent years.