Byte extract with negative offset must not fail invariant

[tautschnig]

The code has provisions for handling negative offsets in a byte extract,
but then tried to convert only parts of index to an unsigned number: the
overall index is known to be non-negative in this branch, but the offset
alone may still be negative.

• 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 #5820 (0255dd3) into develop (3cb60fc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5820   +/-   ##
========================================
  Coverage    69.83%   69.83%           
========================================
  Files         1242     1242           
  Lines       100842   100842           
========================================
  Hits         70422    70422           
  Misses       30420    30420           

Flag	Coverage Δ
cproversmt2	43.61% <0.00%> (ø)
regression	66.79% <100.00%> (ø)
unit	32.28% <0.00%> (ø)

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

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_byte_extract.cpp	72.58% <100.00%> (ø)

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