Use correct type for pointer checks in contracts

[SaswatPadhi]

Resolves #6106.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[feliperodri]

Perfect! 👯

[codecov]

Codecov Report

Merging #6164 (09b87ec) into develop (538070c) will increase coverage by 0.09%.
The diff coverage is 100.00%.

❗ Current head 09b87ec differs from pull request most recent head 45cb253. Consider uploading reports for the commit 45cb253 to get more accurate results

@@             Coverage Diff             @@
##           develop    #6164      +/-   ##
===========================================
+ Coverage    74.95%   75.04%   +0.09%     
===========================================
  Files         1450     1450              
  Lines       158274   158274              
===========================================
+ Hits        118632   118782     +150     
+ Misses       39642    39492     -150     

Impacted Files	Coverage Δ
src/goto-instrument/code_contracts.cpp	85.40% <100.00%> (ø)
src/util/expr.cpp	74.01% <0.00%> (-0.79%)	⬇️
src/util/simplify_expr.cpp	84.71% <0.00%> (-0.47%)	⬇️
src/ansi-c/expr2c.cpp	69.10% <0.00%> (-0.10%)	⬇️
src/util/json.cpp	87.38% <0.00%> (+1.80%)	⬆️
src/ansi-c/c_typecheck_type.cpp	76.38% <0.00%> (+12.84%)	⬆️
src/ansi-c/ansi_c_entry_point.cpp	87.44% <0.00%> (+21.39%)	⬆️

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