Fix buffered iterator

[salvacorts]

What this PR does / why we need it:

In #9700, we added support for writing non-indexed labels from entries into chunks. This PR introduced two bugs:

• When the buffered iterator is closed, the buffer to read metadata labels is put back into the pool but not set to nil. Subsequent uses of the iterator may write on top of the buffer that was put back on the pool. This may lead to inconsistent/incorrect results.
• Inside the buffered iterator, we were not correctly handling EOFs while reading the number of labels and each label length. This was due to the lastAttemp variable not being reset.

This PR adds a new test for these two bugs and also fixes them.

Which issue(s) this PR fixes:
Fixes #9700

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR

[sandeepsukhani]

LGTM

[vlad-diachenko]

LGTM