message: fix BiCGSTAB half iteration log
diff: @@ -194,7 +194,7 @@ void Bicgstab<ValueType>::apply_impl(const LinOp *b, LinOp *x) const
  &stop_status));
  }
  this->template log<log::Logger::iteration_complete>(this, iter,
  - r.get());
  + s.get());
  if (all_converged) {
  break;
  }
  