It is one of the 20th century’s most memorable images: a naked girl, screaming, running from a napalm bombing during the Vietnam War. More than a half-century later, a new documentary is calling into question who took it — and the retired Associated Press photographer long credited for the photo insists it was his, while his longtime employer says it has no evidence of anyone else being behind the camera. The film about the Pulitzer Prize-winning picture, “The Stringer,” is scheduled to debut next week at the Sundance Film Festival. Both photographer Nick Ut and his longtime employer are contesting