The highest court in Massachusetts has rejected a request for a new trial from a man convicted of fatally stabbing a convenience store clerk. Edwin Mejia of Everett was convicted in the 2005 killing of Lourdes Hernandez at the LukOil Mini-Mart in Dorchester. In his appeal, Mejia argued that the prosecutor at his trial made inappropriate statements during his closing argument and that the judge gave a defective instruction to the jury on reasonable doubt. On Tuesday, the Supreme Judicial Court rejected those arguments and affirmed his conviction. Mejia also worked at the store, but had been fired from his job about six months before the killing. Prosecutors said he beat and stabbed Hernandez during a robbery attempt.