Fix ClientSubnetv4 Calculates Address Length Incorrectly issue

[wenxuan70]

Checklist:

• [ x ] If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• [ x ] I squashed commits belonging together
• [ x ] I added unit tests or explained why they are not relevant
• [ x ] I executed the regression tests (using tox)
• [ x ] If the PR is still not finished, please create a Draft Pull Request

fixes #4920

[guedou]

Thanks for this PR. This looks good to me!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.10%. Comparing base (ded1d73) to head (8789985).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4922   +/-   ##
=======================================
  Coverage   80.10%   80.10%           
=======================================
  Files         370      370           
  Lines       91727    91727           
=======================================
+ Hits        73477    73481    +4     
+ Misses      18250    18246    -4     

Files with missing lines	Coverage Δ
scapy/layers/dns.py	83.61% <100.00%> (ø)

... and 4 files with indirect coverage changes

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