`overlay` panics on valid utf-8 input

### Describe the bug

```
  SELECT overlay('aébc' placing 'X' from 3 for 1);
-- Panics; correct result is 'aéXc'

  SELECT overlay('aébc' placing 'ZZ' from 2 for 2);
-- Returns 'aZZbc'; correct result is 'aZZc'
```

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_