detect/dsize: define offset in edge case

[catenacyber]

Link to ticket: https://redmine.openinfosecfoundation.org/issues/
https://redmine.openinfosecfoundation.org/issues/7802

Describe changes:

• detect/dsize: define offset in edge case

Avoids memsan error

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.71%. Comparing base (2a17ab6) to head (d092201).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13769      +/-   ##
==========================================
- Coverage   83.72%   83.71%   -0.01%     
==========================================
  Files        1011     1011              
  Lines      275068   275069       +1     
==========================================
- Hits       230309   230284      -25     
- Misses      44759    44785      +26     

Flag	Coverage Δ
fuzzcorpus	62.98% <100.00%> (+0.03%)	⬆️
livemode	19.00% <0.00%> (-0.01%)	⬇️
pcap	44.73% <100.00%> (-0.01%)	⬇️
suricata-verify	65.08% <100.00%> (+0.01%)	⬆️
unittests	59.18% <100.00%> (+<0.01%)	⬆️

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[suricata-qa]

Information: QA ran without warnings.

Pipeline = 27286

[victorjulien]

Merged in #13772, thanks!