Handle corner case

This would in particular lead to NaN corrections if all nodes are
ignored.

dune/solvers/iterationsteps/cgstep.cc

@@ -35,6 +35,10 @@ void CGStep<MatrixType, VectorType>::preprocess()
 template <class MatrixType, class VectorType>
 void CGStep<MatrixType, VectorType>::iterate()
 {
+    // Avoid divide-by-zero. If r_squared was zero, we're done anyway.
+    if (r_squared_old_ <= 0)
+        return;
+
     VectorType q(*x_);
 
     matrix_->mv(p_, q);                             // q_0     = Ap_0