NEW YORK — A Bronx man had just slashed his girlfriend and was threatening to cut his mother’s throat when a police officer fatally shot him from five feet away, New York Police Department brass said Saturday. “These officers were met with a chaotic, fast moving situation, but they had great composure,” NYPD Chief of Patrol John Chell said at a press conference outside the shooting scene, an apartment building in Bedford Park. Chell said the officers’ actions “saved the life” of the knife-wielding man’s mother. Sources said she is 45 or 46 years old. A 911 call summoned two 52nd Precinct offic…