SPRINGFIELD — The state’s top court ruled Thursday that a 50-year sentence given to a teenager was unconstitutional.Dimitri Buffer was convicted in Cook County on four counts of first-degree murder when he was 16. His case will now go back to the Cook County Circuit Court for resentencing.Justice P. Scott Neville Jr. delivered the opinion and Justice Anne M. Burke specially concurred with her own opinion.Neville wrote the sentence handed down by Circuit Judge Thaddeus L. Wilson violated the Eighth Amendment …