The error in groupConcatOrder/topNRows cannot be accessed from outside

[YangKeao]

The only assignment to the (topNRows).err is:

tidb/pkg/executor/aggfuncs/func_group_concat.go

Lines 323 to 330 in 6a35d5e

	func (h topNRows) Less(i, j int) bool {
	n := len(h.rows[i].byItems)
	for k := 0; k < n; k++ {
	ret, err := h.rows[i].byItems[k].Compare(h.sctx.TypeCtx(), h.rows[j].byItems[k], h.collators[k])
	if err != nil {
	h.err = err
	return false
	}

However, the topNRows is not a reference receiver, assigning to h.err is meaningless. The err here is actually not returned outside.

This issue exists from the very beginning #16591

[YangKeao]

I found this issue by enabling unusedwrite linter.

[yibin87]

/severity minor