Avoid division by zero convergence rate

dune/solvers/solvers/loopsolver.cc

@@ -110,7 +110,8 @@ void ::LoopSolver<VectorType, BitVectorType>::solve()
         // Please don't replace this call to 'diff' by computing the norm of the difference.
         // In some nonlinear DD applications the 'diff' method may be nonlinear.
         real_type normOfCorrection = this->errorNorm_->diff(oldSolution,iterationStep_->getSol());
-        real_type convRate = normOfCorrection / normOfOldCorrection;
+        real_type convRate = (normOfOldCorrection > 0)
+            ? normOfCorrection / normOfOldCorrection : 0.0;
         error = normOfCorrection;
         normOfOldCorrection = normOfCorrection;
 
@@ -118,7 +119,7 @@ void ::LoopSolver<VectorType, BitVectorType>::solve()
         if (referenceSolution_)
         {
             normOfError = this->errorNorm_->diff(iterationStep_->getSol(), *referenceSolution_);
-            convRate = normOfError / normOfOldError;
+            convRate = (normOfOldError > 0) ? normOfError / normOfOldError : 0.0;
             error = normOfError;
             normOfOldError = normOfError;
         }