Matthew McConaughey’s not pigging out to gain back weight after dropping nearly 50 pounds for a film role as a man with AIDS in “The Dallas Buyers Cub.” The hunky actor’s been on a cautious reverse diet at Sundance while debuting another movie, “Mud.” “Matthew is on a special diet to put on weight,” said a source who saw the actor order lunch at star-studded Nikki Beach Sundance on Saturday. “He’s eating fish because it’s healthier.” Also spotted at Nikki Beach in its debut year at Sundance were Alicia Keys, Jennifer Hudson and Keri Russell. Later, at an intimate dinner for “Mud” at the Grey Goose Blue Door Lounge, McConaughey was seen with a full glass of vino.