feat: miner: defensive check for equivocation

[arajasek]

Related Issues

@Stebalien was concerned that this could cause miners to fail to produce blocks when if they see a reorg of height > 1 partway through the mining loop. The issue is that if the tipset at the height of the base's parent changes, the intersection between old and new base MUST be null, and the miner will fail.

Proposed Changes

Weaken the check against equivocation a little -- if the intersection is null, just use the old base (even though it risks "allowing" some equivocation.).

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• If the PR affects users (e.g., new feature, bug fix, system requirements change), update the CHANGELOG.md and add details to the UNRELEASED section.
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[Stebalien]

I think we need to go with this for now, but we'll need to fix it properly later.