Handle the case of a zero-correction

This happens for mu = 0 e.g.

dune/tectonic/myblockproblem.hh

@@ -77,6 +77,8 @@ template <class MyConvexProblemTypeTEMPLATE> class MyBlockProblem {
     VectorType v = projected_v;
 
     double const vnorm = v.two_norm();
+    if (vnorm == 0) // This can (and needs to be able to) be very small
+      return 0.0;
 
     v /= vnorm; // Rescale for numerical stability