Fix SECURITY_DESCRIPTOR computation

[gpotter2]

• implement offset computation for SECURITY_DESCRIPTOR
• test it

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.01%. Comparing base (be3e1ae) to head (305efc2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/modules/ldaphero.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4809   +/-   ##
=======================================
  Coverage   81.01%   81.01%           
=======================================
  Files         365      365           
  Lines       89069    89075    +6     
=======================================
+ Hits        72155    72165   +10     
+ Misses      16914    16910    -4     

Files with missing lines	Coverage Δ
scapy/layers/smb2.py	88.49% <100.00%> (+0.38%)	⬆️
scapy/modules/ldaphero.py	0.00% <0.00%> (ø)

... and 8 files with indirect coverage changes

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