Fix an integer overflow occurring when a tag has too many fields

[masatake]

Reported-by: Arthur Chan arthur.chan@adalogics.com from Ada Logics
in collaboration with Claude and Anthropic Research.

TODO:

• release 0.5.0
• propagate to ctags repo
• propagate to python-ctags3 repo

[codecov]

Codecov Report

❌ Patch coverage is 68.29268% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.02%. Comparing base (ea0aa55) to head (4086de9).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/test-fix-too-many-fields.c	60.60%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   78.15%   79.02%   +0.86%     
==========================================
  Files          16       17       +1     
  Lines        1703     1740      +37     
  Branches      154      155       +1     
==========================================
+ Hits         1331     1375      +44     
+ Misses        372      365       -7     

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