fix(BUX-474): prevent panics in UnReserveUtxos and VerifyMerkleRoots

[chris-4chain]

Pull Request Checklist

• 📖 I created my PR using provided : CODE_STANDARDS
• 📖 I have read the short Code of Conduct: CODE_OF_CONDUCT
• 🏠 I tested my changes locally.
• ✅ I have provided tests for my changes.
• 📝 I have used conventional commits.
• 📗 I have updated any related documentation.
• 💾 PR was issued based on the Github or Jira issue.

This is a quick fix to prevent nil pointer dereferences, but a more comprehensive solution may be considered in the future.

[mergify]

Welcome to our open-source project @chris-4chain! 💘

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (ba7bb1b) 53.31% compared to head (3919f38) 53.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
- Coverage   53.31%   53.29%   -0.02%     
==========================================
  Files         110      110              
  Lines       10840    10844       +4     
==========================================
  Hits         5779     5779              
- Misses       4596     4600       +4     
  Partials      465      465              

Flag	Coverage Δ
unittests	53.29% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
action_utxo.go	58.09% <0.00%> (ø)
chainstate/merkle_root.go	0.00% <0.00%> (ø)

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