Array theory: gracefully handle negative out-of-bounds access

[tautschnig]

Attempts to convert an access at a constant negative (!) index of an
array constant should just lead to the access being ignored. This
situation was already handled for a positive out-of-bounds case, but
should really be considered in both directions.

• 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 #6772 (f769397) into develop (3108deb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6772   +/-   ##
========================================
  Coverage    76.90%   76.90%           
========================================
  Files         1590     1590           
  Lines       183965   183965           
========================================
+ Hits        141479   141480    +1     
+ Misses       42486    42485    -1     

Impacted Files	Coverage Δ
src/goto-programs/resolve_inherited_component.h	100.00% <ø> (ø)
src/solvers/flattening/boolbv_map.h	100.00% <ø> (ø)
src/util/expr.h	97.22% <ø> (ø)
src/solvers/flattening/arrays.cpp	81.45% <100.00%> (+0.26%)	⬆️

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