fix: Defer closing blocks iter after checking error from loadWorkForGap

[salvacorts]

What this PR does / why we need it:

Compactors may panic if SimpleBloomController.loadWorkForGap returns an error due to setting the defer function that closes the block iterators (blocksIter) which is nil before checking and returning on error.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
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[ashwanthgoli]

lgtm