Use vector::at() rather than operator[] (backport #1735)

[mergify]

This is an automatic backport of pull request #1735 done by Mergify.
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Changes to be committed:
	modified:   include/exiv2/value.hpp
	modified:   src/crwimage_int.cpp
	modified:   src/exif.cpp
	new file:   test/data/issue_1706_poc.exv
	new file:   tests/bugfixes/github/test_issue_1706.py

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

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[codecov]

Codecov Report

Merging #1741 (1e57a35) into main (fe83c73) will increase coverage by 0.01%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##             main    #1741      +/-   ##
==========================================
+ Coverage   66.89%   66.90%   +0.01%     
==========================================
  Files         151      151              
  Lines       20754    20789      +35     
==========================================
+ Hits        13883    13909      +26     
- Misses       6871     6880       +9     

Impacted Files	Coverage Δ
src/value.cpp	80.33% <58.33%> (+0.06%)	⬆️
include/exiv2/value.hpp	90.62% <75.00%> (ø)
src/crwimage_int.cpp	87.04% <100.00%> (ø)
src/exif.cpp	76.10% <100.00%> (+0.26%)	⬆️
src/preview.cpp	57.46% <0.00%> (-0.29%)	⬇️
samples/Jzon.cpp	33.39% <0.00%> (-0.18%)	⬇️
src/webpimage.cpp	64.97% <0.00%> (-0.17%)	⬇️
src/pgfimage.cpp	55.93% <0.00%> (-0.11%)	⬇️
src/image.cpp	72.51% <0.00%> (-0.05%)	⬇️
src/http.cpp	0.00% <0.00%> (ø)
... and 13 more

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[hassec]

Same as in #1735 plus small merge conflict fix -> Approving.

thanks again @kevinbackhouse