Initialise all POD members of goto_symex_statet

[tautschnig]

The constructor did not take care of them and our unit tests exposed that, at least within unit tests, we were accessing uninitialised members.

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[Copilot]

Pull request overview

This PR fixes undefined behavior in unit tests by ensuring previously uninitialized POD members of goto_symex_statet are deterministically initialized.

Changes:

• Initialize symex_target to nullptr in-class.
• Initialize jump-target/next-instruction state flags to false.
• Initialize run_validation_checks to false.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (7a4df92) to head (29c23f5).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8865   +/-   ##
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  Coverage    80.01%   80.01%           
========================================
  Files         1703     1703           
  Lines       188396   188396           
  Branches        73       73           
========================================
  Hits        150738   150738           
  Misses       37658    37658           

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