strtol must not overflow

[tautschnig]

The lack of overflow checking in the internal library first showed up in
SV-COMP when running sv-benchmarks/c/busybox-1.22.0/ls-incomplete-2.i
with --unwind 21.

• 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 #5632 (31147d7) into develop (2858b18) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5632   +/-   ##
========================================
  Coverage    69.36%   69.36%           
========================================
  Files         1241     1241           
  Lines       100564   100564           
========================================
  Hits         69753    69753           
  Misses       30811    30811           

Flag	Coverage Δ
cproversmt2	43.15% <ø> (ø)
regression	66.26% <ø> (ø)
unit	32.27% <ø> (ø)

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