NEW YORK — A 31-year-old man was fatally shot early Tuesday as he guarded a “Law & Order” film set in Brooklyn, police said. The unidentified victim was providing security and guarding equipment trucks for the “Law & Order: Organized Crime” film crew on N. Henry St. near Norman Ave. in Greenpoint around 5:15 a.m. when the gunman approached, the sources said. The victim was sitting in a car — with a traffic cone perched on the roof — when the shooter yanked open the door and shot the victim in the neck and head. “It happened so quick, too fast. I don’t even know who would do this or why or what…