Fix decompression of 6LowPan src

[gpotter2]

fixes #4789

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.02%. Comparing base (0f7b0c0) to head (4cb1dc5).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4792   +/-   ##
=======================================
  Coverage   81.02%   81.02%           
=======================================
  Files         365      365           
  Lines       89068    89072    +4     
=======================================
+ Hits        72170    72174    +4     
  Misses      16898    16898           

Files with missing lines	Coverage Δ
scapy/layers/sixlowpan.py	86.67% <100.00%> (+0.09%)	⬆️

... and 4 files with indirect coverage changes

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