Fix out-of-bounds read in CrwMap::decode0x0805

[kevinbackhouse]

I've created https://github.com/Exiv2/exiv2/security/advisories/GHSA-9mxq-4j5g-5wrp for this, but it's a very minor bug and I'm only able to reproduce with our fuzz target, not with the exiv2 command line application. When creating the string, CrwMap::decode0x0805 might read off the end of the buffer. It stops reading when it gets to the first '\0' byte.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.98%. Comparing base (7465595) to head (5b8f1f4).
⚠️ Report is 3 commits behind head on 0.28.x.

Additional details and impacted files

@@            Coverage Diff             @@
##           0.28.x    #3462      +/-   ##
==========================================
+ Coverage   64.97%   64.98%   +0.01%     
==========================================
  Files         104      104              
  Lines       22261    22260       -1     
  Branches    10878    10878              
==========================================
+ Hits        14463    14465       +2     
+ Misses       5544     5541       -3     
  Partials     2254     2254              

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