Use replacement value for NAN %

[Spacetown]

A function returned value of NAN %is replaced by 0.

Fixed #908

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 94.57%. Comparing base (9172209) to head (ad07200).

Files	Patch %	Lines
gcovr/formats/gcov/parser.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #910      +/-   ##
==========================================
- Coverage   94.61%   94.57%   -0.05%     
==========================================
  Files          50       50              
  Lines        3884     3889       +5     
  Branches      814      816       +2     
==========================================
+ Hits         3675     3678       +3     
- Misses        130      131       +1     
- Partials       79       80       +1     

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