The cause of death for “ Buffy the Vampire Slayer ” actor Nicholas Brendon has been revealed, nearly two months after he died at 54. According to a report from the coroner’s office in Putnam County, N.Y., obtained by People , Brendon died of atherosclerotic and hypertensive cardiovascular disease. Pneumonia and previous myocardial infarction were listed as contributing factors, and the “mechanism of death” was 90% blockage in the right coronary artery. Brendon’s family had said in a statement on March 20 that he had died in his sleep of natural causes. “We are heartbroken to share the passing of our brother and son, Nicholas Brendon,” his family wrote at the time. “He passed in his sleep of natural causes. Most people know Nicky for his work as an actor and for the characters he brought to life over the years. In recent years Nicky has found his passion in painting and art. Nicky loved to share his enthusiastic talent with his family, friends and fans.…