Fix IPFIX seconds timestamp field lengths

[guedou]

This PR fixes #4198

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.15%. Comparing base (e2e35c0) to head (8c2b734).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5225      +/-   ##
==========================================
+ Coverage   81.14%   81.15%   +0.01%     
==========================================
  Files         393      393              
  Lines       98325    98325              
==========================================
+ Hits        79786    79796      +10     
+ Misses      18539    18529      -10     

Files with missing lines	Coverage Δ
scapy/layers/netflow.py	91.84% <ø> (+0.60%)	⬆️

... and 6 files with indirect coverage changes

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