Fix xdg folder permission check for cases when privileges were dropped

[Civil]

If privileges for scapy were dropped, but username remain unchanged, path.exist() would trigger an exception. Fix that by moving whole if statement under try-except.

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
  Tests are not relevant because it is extremely hard to write a proper test for that behaviour.
• 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 #4618

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.71%. Comparing base (cb4a95b) to head (83adf2b).
Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/main.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4619      +/-   ##
==========================================
- Coverage   81.31%   78.71%   -2.61%     
==========================================
  Files         359      334      -25     
  Lines       86047    80848    -5199     
==========================================
- Hits        69971    63641    -6330     
- Misses      16076    17207    +1131     

Files with missing lines	Coverage Δ
scapy/main.py	69.13% <50.00%> (-0.87%)	⬇️

... and 280 files with indirect coverage changes

[gpotter2]

Thanks for the PR !