It may sound harsh, but as a dog owner, you have the responsibility to keep your animal under control, particularly something as intimidating as a bull mastiff. If you can't keep it under control, as in this lady's case, then don't take it out in public.
Your dog may be the sweetest thing ever, but I don't know that and it's not on me to figure it out as it comes running up. If it attacks me or mine, then I'm going to defend myself, and if I'm carrying a gun, it'll likely get shot.
Bottom line - the dog owner is responsible for controlling the dog, if they can't control it, then don't own it.
Just remember, when you pull the trigger, the bullets come out going very, very fast. So make sure to keep the weapon pointing away from you.