Fix subtraction with overflow panic when parse malformed DTD

[Mingun]

Initially fix is implemented in v0.39 branch, that is started from v0.39.2 tag, and then merged in issue950 branch (the branch that is PRed to master). I plan also include this fix into v0.39.3 patch (because master contains some breaking changes, that cannot be included in v0.39.3).

Closes #950

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.40%. Comparing base (a759d65) to head (c05c784).
⚠️ Report is 13 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #954      +/-   ##
==========================================
+ Coverage   55.08%   56.40%   +1.31%     
==========================================
  Files          44       44              
  Lines       16911    17606     +695     
==========================================
+ Hits         9316     9931     +615     
- Misses       7595     7675      +80     

Flag	Coverage Δ
unittests	56.40% <100.00%> (+1.31%)	⬆️

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