kerberos: fix S4U when using FAST

[gpotter2]

fixes #5064

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.81%. Comparing base (1d29984) to head (9ca6c16).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/kerberos.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5165      +/-   ##
==========================================
- Coverage   80.85%   80.81%   -0.04%     
==========================================
  Files         390      390              
  Lines       97734    97736       +2     
==========================================
- Hits        79021    78985      -36     
- Misses      18713    18751      +38     

Files with missing lines	Coverage Δ
scapy/layers/kerberos.py	71.79% <0.00%> (+0.10%)	⬆️

... and 30 files with indirect coverage changes

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