Investigators believe that the veteran NYPD cop wounded during a chaotic confrontation with a gun-toting teen Sunday was struck by friendly fire, law-enforcement sources told The Post. Detective Robert Karroll was hit in the back by a slug — and only saved from potentially deadly injuries by his bulletproof vest — around 4:15 a.m. when a deranged 18-year-old ran up on an unmarked police car in Crown Heights, authorities have said. Karroll, a married dad of three who was just 10 days from retirement at the time, was in the vehicle along with three other officers — all of whom...