Replace assertion with an error message (backport #1848)

[mergify]

This is an automatic backport of pull request #1848 done by Mergify.
Cherry-pick of e33a2be has failed:

On branch mergify/bp/main/pr-1848
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit e33a2bee.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   test/data/issue_1847_poc.jpg
	new file:   tests/bugfixes/github/test_issue_1847.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/tiffcomposite_int.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[codecov]

Codecov Report

Merging #1849 (9e160a4) into main (cca3e07) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1849      +/-   ##
==========================================
- Coverage   60.62%   60.61%   -0.01%     
==========================================
  Files          96       96              
  Lines       18873    18874       +1     
  Branches     9487     9488       +1     
==========================================
  Hits        11441    11441              
- Misses       5144     5145       +1     
  Partials     2288     2288              

Impacted Files	Coverage Δ
src/tiffcomposite_int.cpp	73.85% <0.00%> (-0.08%)	⬇️

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