Return zero at zero (for gradient/hessian)

dune/tectonic/localnonlinearity.hh

@@ -62,6 +62,9 @@ template <int dimension> class LocalNonlinearity {
     = (h2 - h1ox) * x[i] * x[j] / normX2
   */
   void addHessian(VectorType const &x, MatrixType &A) const {
+    if (x == VectorType(0))
+      return;
+
     double const normX2 = x.two_norm2();
     double const normX = sqrt(normX2);
     double const h1ox = func_->rightDifferential(normX) / normX;
@@ -79,6 +82,9 @@ template <int dimension> class LocalNonlinearity {
   }
 
   void addGradient(VectorType const &x, VectorType &y) const {
+    if (x == VectorType(0))
+      return;
+
     VectorType tmp;
     upperGradient(x, tmp); // TODO
     y += tmp;