CONTRACTS: Fix conditional havoc code

[remi-delmas-3000]

Fixes #7379

The condition was being checked on the write set variable instead of write set element variable

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• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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[feliperodri]

LGTM modulo a few clean ups.

[codecov]

Codecov Report

Base: 78.35% // Head: 78.36% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (6b2a4ee) compared to base (c7550b6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7380   +/-   ##
========================================
  Coverage    78.35%   78.36%           
========================================
  Files         1645     1645           
  Lines       190182   190247   +65     
========================================
+ Hits        149015   149080   +65     
  Misses       41167    41167           

Impacted Files	Coverage Δ
...t/contracts/dynamic-frames/dfcc_spec_functions.cpp	73.07% <100.00%> (-0.15%)	⬇️
src/goto-checker/symex_coverage.cpp	96.35% <0.00%> (ø)
src/goto-programs/goto_convert_class.h	88.15% <0.00%> (ø)
src/goto-checker/single_path_symex_checker.h	100.00% <0.00%> (ø)
src/goto-programs/goto_convert_functions.cpp	100.00% <0.00%> (ø)
src/goto-checker/multi_path_symex_checker.cpp	95.65% <0.00%> (ø)
src/goto-checker/single_path_symex_only_checker.h	100.00% <0.00%> (ø)
src/goto-checker/multi_path_symex_only_checker.cpp	100.00% <0.00%> (ø)
...rc/goto-checker/single_path_symex_only_checker.cpp	97.53% <0.00%> (ø)
...-checker/single_loop_incremental_symex_checker.cpp	90.82% <0.00%> (ø)
... and 3 more

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