Dr. Hannibal Lecter said:
for a cheap way to do it.
1. Make a frame the same size as you miror that is coated with a wax realease agent.
2. Place mylar under the frame. Get it ar a survival store.
3. make some sugar glass and fill the frame resipes are on the net.
4. Carfully remove the frame. And place the brake away glass where the mirror was.
When the actor hits the glass it will shatter without huring the actor.
Another way is to film the actor from the side or front punching the mirror and adding the sound of it breaking in post and cut to a mirror that is already broken as the actor removes his hand.