Use "replace" error strategy for utf-8 decoding

[gierdo]

As previously discussed in #214

This PR makes _CK_UTF8CHAR_to_str more robust against non-compliant tokens.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.90%. Comparing base (fe57001) to head (aa64ba6).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   89.86%   89.90%   +0.03%     
==========================================
  Files          15       15              
  Lines        2604     2604              
  Branches       29       29              
==========================================
+ Hits         2340     2341       +1     
+ Misses        261      260       -1     
  Partials        3        3              

Flag	Coverage Δ
unittests	89.90% <100.00%> (+0.03%)	⬆️

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