Check for |u| = 0 instead of u = 0

If |u| is computed as 0, we should not divide by it (this appears to
happen even for u != 0)

src/mynonlinearity.hh

@@ -32,7 +32,7 @@ class MyNonlinearity {
   // u and v are assumed to be non-zero
   void directionalSubDiff(VectorType const u, VectorType const v,
                           Interval<double> &D) const {
-    if (u == SmallVector(0.0)) {
+    if (u.two_norm() == 0) {
       D[0] = D[1] = func_.rightDifferential(0) * v.two_norm();
       return;
     }