fix: bounds-check hexdiff() offset-skip loop to prevent IndexError

[citizen204]

Summary

hexdiff() raises IndexError: list index out of range when the difflib alignment (algo='difflib') produces a 16-byte display row that is entirely a "gap" on one side (a run of >=16 aligned insertions/deletions). The loop that skips leading empty cells to compute the row's starting offset had no bounds check and walked off the end of the row.

Fixes #5034

Changes

• scapy/utils.py: added j < len(linex) / j < len(liney) bounds checks to the two offset-skip while loops in hexdiff(), so the loop stops once it reaches the end of a fully-gapped row instead of indexing past it.
• test/regression.uts: added a regression test using the minimal reproduction from the issue (hexdiff(b"B"*9 + b"\x08", b"A" + b"\x08" + b"A"*23, algo="difflib")), which previously raised IndexError and now completes without crashing.

AI-Assisted: yes (Claude Sonnet 5)

[polybassa]

Thanks for the fix

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.69%. Comparing base (f967117) to head (d8e8d29).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5036      +/-   ##
==========================================
- Coverage   80.21%   79.69%   -0.52%     
==========================================
  Files         387      387              
  Lines       96405    96405              
==========================================
- Hits        77331    76831     -500     
- Misses      19074    19574     +500     

Files with missing lines	Coverage Δ
scapy/utils.py	72.03% <100.00%> (-0.05%)	⬇️

... and 22 files with indirect coverage changes

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