Avoid division by 0 in NewtonFool

Author: Irina Nicolae

art/attacks/newtonfool.py

@@ -119,8 +119,10 @@ def _compute_pert(theta, grads, norm_grad):
         :param norm_grad: norm of gradient values at the attacked class.
         :return: pertubation.
         """
+        # Pick a small scalar to avoid division by 0
+        tol = 10e-8
         nom = -theta * grads
-        denom = norm_grad**2
+        denom = norm_grad**2 if norm_grad > tol else tol
         result = nom / float(denom)
 
         return result