Check that the iterator is not end() after calling findKey (backport #1758)

[mergify]

This is an automatic backport of pull request #1758 done by Mergify.
Cherry-pick of 18d1689 has failed:

On branch mergify/bp/main/pr-1758
Your branch is ahead of 'origin/main' by 1 commit.
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You are currently cherry-picking commit 18d16895.
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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/pentaxmn_int.cpp

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 3ac426b has failed:

On branch mergify/bp/main/pr-1758
Your branch is ahead of 'origin/main' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 3ac426b2.
  (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:
	modified:   src/crwimage_int.cpp
	modified:   src/exif.cpp
	modified:   src/iptc.cpp
	modified:   src/xmp.cpp

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

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[codecov]

Codecov Report

Merging #1761 (b457bb3) into main (d30c95d) will increase coverage by 0.06%.
The diff coverage is 97.36%.

@@            Coverage Diff             @@
##             main    #1761      +/-   ##
==========================================
+ Coverage   67.22%   67.29%   +0.06%     
==========================================
  Files         151      151              
  Lines       20787    20797      +10     
==========================================
+ Hits        13975    13995      +20     
+ Misses       6812     6802      -10     

Impacted Files	Coverage Δ
src/convert.cpp	69.32% <92.30%> (+0.11%)	⬆️
src/crwimage_int.cpp	87.16% <100.00%> (+0.12%)	⬆️
src/exif.cpp	76.10% <100.00%> (ø)
src/iptc.cpp	76.05% <100.00%> (ø)
src/pentaxmn_int.cpp	83.94% <100.00%> (+1.23%)	⬆️
src/xmp.cpp	90.55% <100.00%> (ø)
src/tiffcomposite_int.cpp	83.12% <0.00%> (+0.90%)	⬆️

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