Casting teen dream Matt Dillon as a junkie going straight sounds like one of those noxious notions dreamed up by a politician for yet another Just Say No campaign. Admirable causes usually make for deadly moviemaking. The film's beginning suggests this is no exception. Dillon's Bob Hughes, shot by a kid pusher, is near death. Bob narrates the story in a long flashback, waxing poetic about his days of robbing drugstores for a fix. Hearing Dillon's thick tongue trip over such twisters as "the rosy…