A couple who maintain their son was denied a fair trial when he was found guilty of murdering his wife and children do not have a case for violation of their own constitutional rights, a federal judge held.In a written opinion, U.S. District Judge Manish S. Shah of the Northern District of Illinois did not rule on the merits of Pierre and Gail Vaughn’s argument that law enforcement officers and prosecutors obtained their son’s conviction using false testimony and invalid forensic findings.Instead, Shah held the Vaughns …