Replace assertion with error message in TiffDirectory::writeDirEntry() (backport #1846)

[mergify]

This is an automatic backport of pull request #1846 done by Mergify.

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/

[codecov]

Codecov Report

Merging #1852 (15baaca) into main (66259b1) will increase coverage by 0.07%.
The diff coverage is 33.33%.

❗ Current head 15baaca differs from pull request most recent head 9fd8b40. Consider uploading reports for the commit 9fd8b40 to get more accurate results

@@            Coverage Diff             @@
##             main    #1852      +/-   ##
==========================================
+ Coverage   60.70%   60.78%   +0.07%     
==========================================
  Files          96       96              
  Lines       18877    18879       +2     
  Branches     9491     9493       +2     
==========================================
+ Hits        11460    11475      +15     
+ Misses       5124     5114      -10     
+ Partials     2293     2290       -3     

Impacted Files	Coverage Δ
src/tiffcomposite_int.cpp	74.40% <33.33%> (+0.47%)	⬆️
src/exiv2.cpp	57.85% <0.00%> (+0.22%)	⬆️
src/tiffvisitor_int.cpp	75.84% <0.00%> (+0.33%)	⬆️
src/jp2image.cpp	69.46% <0.00%> (+0.69%)	⬆️
src/tiffimage.cpp	69.53% <0.00%> (+0.78%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 66259b1...9fd8b40. Read the comment docs.