NICHOLAS Sparks has sold so many books, he worries about the safety of his wife and children. So he has two German shepherds, Rex and Laura, who have been trained from puppyhood as guard dogs. Sparks — whose “The Last Song” is No. 1 on best-seller lists and which Disney is bringing out as a film in January starring Miley Cyrus — told literary guru Lewis Burke Frumkes at Meli Melo restaurant the other day the dogs are so well-trained that if Nick says “Attend,” Rex and Laura will hold any intruder at bay without biting. If he says “Attack,” God help the crook.