A rock cocaine possession charge against one-time rock music star Sly Stone was dismissed after a Los Angeles County Superior Court commissioner ruled that testimony from the two police officers who arrested him was not credible. The officers testified at a pretrial hearing that they had witnessed Stone drop a rock of cocaine from his hand to the floorboard of his car after pulling up outside the Hollywood Bowl Motel last Oct. 23. The prosecution asked that the case be dismissed after Commissioner Florence-Marie Cooper made her ruling, said Deputy Dist. Atty. Sally Thomas. Stone, whose real name is Sylvester Stewart, recorded such hits as "Dance to the Music" in the late 1960s with his group, "Sly and the Family Stone."