NEW YORK (Reuters) - The S&P 500 ended just slightly in the red on Monday, with bank shares falling amid warnings of potential losses from a hedge fund’s default on margin calls, while optimism over the economy limited the day’s declines. The Dow ended higher, with shares of planemaker Boeing Co rising 2.3% after the company reached a deal with U.S. budget carrier Southwest Airlines Co for a variant of the 737 MAX aircraft.