message: Fix for valgrind warnings for Sellp
  In Sellp::write(), a value was read before checking if it is even inside
  the size restrictions, leading to the valgrind warning:
  "Conditional jump or move depends on uninitialised value(s)"
  The fix was simply moving the check before reading a value.
diff: @@ -220,19 +220,22 @@ void Sellp<ValueType, IndexType>::write(mat_data &data) const
  for (size_type row_in_slice = 0; row_in_slice < slice_size;
  row_in_slice++) {
  auto row = slice * slice_size + row_in_slice;
  + if (row < tmp->get_size()[0]) {
  for (size_type i = 0; i < tmp->get_const_slice_lengths()[slice];
  i++) {
  const auto val = tmp->val_at(
  row_in_slice, tmp->get_const_slice_sets()[slice], i);
  - if (val != zero<ValueType>() && row < tmp->get_size()[0]) {
  - const auto col = tmp->col_at(
  - row_in_slice, tmp->get_const_slice_sets()[slice], i);
  + if (val != zero<ValueType>()) {
  + const auto col =
  + tmp->col_at(row_in_slice,
  + tmp->get_const_slice_sets()[slice], i);
  data.nonzeros.emplace_back(row, col, val);
  }
  }
  }
  }
  }
  +}
  #define GKO_DECLARE_SELLP_MATRIX(ValueType, IndexType) \
  