Fix: Expiration bucket check on cover expiration

[shark0der]

Context

Due to a lax check on the expiration bucket id the expire cover could revert with an underflow since the allocations could have been already freed up.

Changes proposed in this pull request

This PR moves corrects the check and moves it earlier in the code to fail with a custom error instead.

Test plan

Added a new test that:

1. Allocates a short cover in the current bucket
2. Moves to the next bucket
3. Tries to expire

Before the fix it will fail with an underflow, with the fix it fails with a custom error.

Checklist

• Rebased the base branch
• Attached corresponding Github issue
• Prefixed the name with the type of change (i.e. feat, chore, test)
• Performed a self-review of my own code
• Followed the style guidelines of this project
• Made corresponding changes to the documentation
• Didn't generate new warnings
• Didn't generate failures on existing tests
• Added tests that prove my fix is effective or that my feature works

Review

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• 🔨 = I believe this should be changed.
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• 🚀 = Feedback

[danoctavian]

LGTM