### Describe Your Changes
evalInstantRollup could have overreport the number of fetched series if
`offset` checks will result into retry. This change updates fetched
series only if these checks were successful.
It also adds a comment to another potential place of over-reporting
series fetched. It doesn't fix it, because it would require spending
extra resources on such a check, while discrepancy in seriesFetched
doesn't affect calculations in any way.
Probably related to
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7170
### Checklist
The following checks are **mandatory**:
- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
Signed-off-by: hagen1778 <roman@victoriametrics.com>
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