"[T]he guilty as well as the innocent are entitled to due process of law. They are entitled to a fair trial. They are entitled to counsel. They are entitled to fair treatment from the police. The law enforcement officer has the same duty as the citizen-indeed, he has a higher duty-to abide by the letter and spirit of our Constitution and laws. You yourselves must be careful to obey the letter of the law. You yourselves must be intellectually honest in the enforcement of the law."