Fix ICMPv6 RFC4884 extension length handling.

[guedou]

This PR fixes #4353

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.14%. Comparing base (e2e35c0) to head (65ef3c8).

Files with missing lines	Patch %	Lines
scapy/layers/inet6.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5222   +/-   ##
=======================================
  Coverage   81.14%   81.14%           
=======================================
  Files         393      393           
  Lines       98325    98342   +17     
=======================================
+ Hits        79786    79802   +16     
- Misses      18539    18540    +1     

Files with missing lines	Coverage Δ
scapy/layers/inet6.py	88.79% <88.23%> (-0.01%)	⬇️

... and 6 files with indirect coverage changes

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