Best of luck to you. It sounded like you acted reasonably--apart from giving too much information to the inexperienced officer.
Perhaps you should have said that you fired to stop the attack, but didn't hit the dog. Missing everything on the planet but the Earth itself is not a crime if the shoot is otherwise lawful. The one piece of information you gave that was too much was that you intended to miss the dog!
Don't talk to the cops!
Wait. I shouldn't be talking to you.