RowToStructByName does not scan into embedded structures

**Describe the bug**
I get "struct doesn't have corresponding row field id" error when I try to do RowToStructByName because I have another field in nested structure with `db:"id"` tag
**To Reproduce**
```go
type Subscription struct {
ID int `db:"id"`
TariffAlias string `db:"tariff_alias"`
Tariff
}

type Tariff struct {
ID int `db:"id"`
Alias string `db:"alias"`
}

pgx.CollectOneRow(rows, pgx.RowToStructByName[Subscription])
```

```sql
select * from subscription as s join tariff t on t.alias = s.tariff_alias;
```
**Expected behavior**
I get the completed structure Subscription
**Actual behavior**
struct doesn't have corresponding row field id

**Version**
- Go: go version go1.21 darwin/amd64
- pgx: github.com/jackc/pgx/v5.3.1

**Additional context**
The problem can be corrected by adding zeroing of the field name in the fieldPosByName function, as a mark that it has already found a match in the structure `fldDescs[i].Name = ""`

Start with the fieldPosByName function mentioned in the issue and reproduce the behavior using the Subscription and Tariff structs and the shown query. Trace how duplicate db:"id" fields are matched; done means RowToStructByName returns the completed nested structure without the missing-row-field error.

Written by the indexing model from the issue text.