add extra defensive checks

[rpreen]

Adds extra checks for empty sets before performing operations such as roulette wheel deletion, match set building and set updating -- these should never happen in practice and would segfault if it did --- if this ever does somehow happen, this will gracefully exit with a message; and tries to satisfy the static analysis tools.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (563167b) 61.89% compared to head (2f7b8c4) 61.86%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   61.89%   61.86%   -0.03%     
==========================================
  Files          62       62              
  Lines        8894     8903       +9     
  Branches     1073     1076       +3     
==========================================
+ Hits         5505     5508       +3     
- Misses       3389     3395       +6     

Files	Coverage Δ
xcsf/clset.c	66.66% <33.33%> (-0.82%)	⬇️

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

40.0% Coverage
0.0% Duplication