A federal judge in Massachusetts on Thursday dismissed a defamation lawsuit against Bill Cosby by an actress who claimed he raped her, ruling that the comedian acted within his rights when he proclaimed himself innocent of the crime. The civil lawsuit is one of a slew of cases brought against the actor, best known for playing the father in the 1980s television hit “The Cosby Show,” by about 50 women who say he sexually assaulted them, often after plying them with drugs and alcohol, in a series of alleged attacks dating back decades.