Bobbi Kristina Brown’s ex-partner died last month from a heroin overdose, an autopsy found.Nick Gordon, whose legal name is Nicholas Bouler, died from an accidental overdose on New Year’s Day, the medical examiner in Seminole County, Florida, found in an autopsy released Thursday.Gordon, 30, was found unresponsive at a Sheraton hotel in the Orlando-area suburb of Maitland, police said.