Offset computation can handle zero-sized objects

[tautschnig]

Although any access would result in an out-of-bounds access (within the
program being verified), it's perfectly fine to perform offset
computation.

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• 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.

[codecov]

Codecov Report

Merging #5693 (6b09359) into develop (95097a8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5693   +/-   ##
========================================
  Coverage    69.49%   69.49%           
========================================
  Files         1243     1243           
  Lines       100674   100674           
========================================
  Hits         69962    69962           
  Misses       30712    30712           

Flag	Coverage Δ
cproversmt2	43.10% <40.00%> (-0.01%)	⬇️
regression	66.38% <100.00%> (ø)
unit	32.25% <40.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_get.cpp	76.82% <100.00%> (+0.15%)	⬆️
src/solvers/flattening/bv_pointers.cpp	82.40% <100.00%> (ø)
src/util/pointer_offset_size.cpp	94.83% <100.00%> (-0.02%)	⬇️

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