Fixed stun packet creation

[gkpln3]

Checklist:

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

fixes #xxx

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.56%. Comparing base (6b26ace) to head (80d1517).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4421      +/-   ##
==========================================
- Coverage   82.21%   81.56%   -0.65%     
==========================================
  Files         352      352              
  Lines       84032    84032              
==========================================
- Hits        69083    68544     -539     
- Misses      14949    15488     +539     

Files	Coverage Δ
scapy/contrib/stun.py	88.46% <100.00%> (+6.41%)	⬆️

... and 23 files with indirect coverage changes

[gpotter2]

Thanks for the PR. Could you add a unit test?
See the test/contribs/ folder

[gkpln3]

Thanks for the PR. Could you add a unit test? See the test/contribs/ folder

Thanks for the comment, I've added some UT