'Heartland' Melodrama by Kevin Heelan. Directed by Art Wolf. A sadistic father (J. C. Quinn), his victimized teen-age son (Sean Penn), and the father's easygoing friend (Larry Nicks) stand guard on the porch of a house in a small Midwestern town. A killer is terrifying the community. Radio bulletins report the mounting toll of victims. Crickets chirp in the darkness. The local sheriff (Keith Jochim) stops by and warns the gun-happy father against taking the law into his own hands. Such are the stock ingredients of Kevin Heelan's "Heartland," the well-acted, tautly written, but essentially trite thriller at the Century Theater. With its one-dimensional characters and inevitable shotgun blasts, the melodrama might get by as a made-for-TV movie. But not as Broa dway fare.