Last week’s stock market sell-off yanked the S&P 500 sharply below its 50-day moving average. As the index tries to rebound, risk assessment leader Moody’s (MCO) investigates a new buy zone. On Nov. 4 — the day after the U.S. midterm elections — shares of Moody’s briefly cleared a 523.31 buy point. While the S&P 500 and Fortune 500 member has…