compute relativeError only if the result isn't nan

[[Imported from SVN: r3249]]

dune/solvers/solvers/loopsolver.cc

@@ -105,7 +105,7 @@ void ::LoopSolver<VectorType, BitVectorType>::solve()
         }
 
         // Turn the error into the relative error, if requested
-        if (this->useRelativeError_)
+        if (this->useRelativeError_ && !std::isnan(error/oldNorm))
             error = error / oldNorm;
 
         if (!isinf(convRate) && !isnan(convRate) && i>0) {