cbmc-cover/pointer-function-parameters test: negative numbers are ok

[tautschnig]

We need to cover three scenarios: a is NULL, or *a is 4, or *a is some integer other than 4. The latter includes negative values.

Seen in
https://github.com/diffblue/cbmc/actions/runs/7740169274/job/21104572343?pr=8141.

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• 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).
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (608b7ea) 79.69% compared to head (99ac9b0) 79.69%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8177   +/-   ##
========================================
  Coverage    79.69%   79.69%           
========================================
  Files         1680     1680           
  Lines       195242   195242           
========================================
+ Hits        155604   155605    +1     
+ Misses       39638    39637    -1     

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