dcerpc: fragment vt_trailer and frag_len properly

[gpotter2]

• vt_trailer should only be present in the last fragment when fragmenting
• frag_len is miscalculated in fragments. this is an issue with header signing enabled

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.82%. Comparing base (1d29984) to head (8e6b3cd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5161      +/-   ##
==========================================
- Coverage   80.85%   80.82%   -0.04%     
==========================================
  Files         390      390              
  Lines       97734    97736       +2     
==========================================
- Hits        79021    78991      -30     
- Misses      18713    18745      +32     

Files with missing lines	Coverage Δ
scapy/layers/dcerpc.py	91.32% <100.00%> (+0.71%)	⬆️

... and 28 files with indirect coverage changes

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