Add enforce to avoid out-of-bounds read

[kevinbackhouse]

Fix for https://github.com/Exiv2/exiv2/security/advisories/GHSA-hxph-pv7w-8649, an out-of-bounds read found by OSS-Fuzz.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.55%. Comparing base (5b8f1f4) to head (c798e33).
⚠️ Report is 52 commits behind head on 0.28.x.

Additional details and impacted files

@@            Coverage Diff             @@
##           0.28.x    #9308      +/-   ##
==========================================
+ Coverage   64.98%   65.55%   +0.57%     
==========================================
  Files         104      104              
  Lines       22260    22293      +33     
  Branches    10878    10901      +23     
==========================================
+ Hits        14465    14614     +149     
+ Misses       5541     5355     -186     
- Partials     2254     2324      +70     

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