Released 50 years ago, “Taxi Driver” tells the story of Travis Bickle (played by Robert De Niro), a cab driver in 1970s New York who projects feelings of fury and indignation onto the city, then at a low ebb. It came amid a wave of films that scrutinized the institutions that once anchored postwar America, including the political system (“The Parallax View”) and the Central Intelligence Agency (“Three Days of the Condor”). But unlike those movies, “Taxi Driver” declined to provide audiences with an access point in the form of a noble protagonist.