imp: added more assertions in verify_header

[srdtrk]

Description

closes: #407
closes: #410

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Wrote unit and integration tests.
• Added relevant natspec and godoc comments.
• Provide a conventional commit message to follow the repository standards.
• Re-reviewed Files changed in the GitHub PR explorer.
• Review SonarCloud Report in the comment section below once CI passes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.86%. Comparing base (b05dea1) to head (c314261).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #415   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          14       14           
  Lines         744      744           
=======================================
  Hits          743      743           
  Misses          1        1           

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[gjermundgaraba]

lgtm!