goto_rw: byte extracts may have a negative offset

[tautschnig]

A negative offset can be safely ignored as there is nothing to be read
from or to be written to.

Fixes: #2524

• 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.
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[codecov]

Codecov Report

Merging #6668 (5c3acff) into develop (673bd73) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff            @@
##           develop    #6668   +/-   ##
========================================
  Coverage    76.76%   76.77%           
========================================
  Files         1582     1582           
  Lines       182357   182358    +1     
========================================
+ Hits        139988   140004   +16     
+ Misses       42369    42354   -15     

Impacted Files	Coverage Δ
src/analyses/goto_rw.cpp	55.79% <66.66%> (+3.49%)	⬆️
src/cbmc/cbmc_parse_options.cpp	77.77% <0.00%> (+0.21%)	⬆️

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[tautschnig]

Closing as #2822 already tackles this issue. Pushing an updated fix to the branch behind #2822 instead.