C library: remove explicit zero initialisers

[tautschnig]

We initialise (with the default zero) value global variables in multiple model functions, which can result in spurious warnings about those variables already having an initial value.

Towards: #8638

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• n/a 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).
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• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.39%. Comparing base (4a1a325) to head (6c354a5).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8698      +/-   ##
===========================================
- Coverage    80.40%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207403   207403              
  Branches        73       73              
===========================================
- Hits        166763   166749      -14     
- Misses       40640    40654      +14     

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