Fixed bug on stat handler

[marcoSanti]

This pull request fixes a bug in the stat syscall, where the application sometimes passes a NULL value to the syscall, expecting it to fail. CAPIO previously always attempted to perform a cast, which caused a segmentation fault. The code now checks for NULL values beforehand and ignores syscalls with invalid parameters.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.37%. Comparing base (779c967) to head (b604a00).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
capio/posix/handlers/stat.hpp	0.00%	5 Missing and 1 partial ⚠️
capio/posix/handlers/statx.hpp	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
- Coverage   56.50%   56.37%   -0.14%     
==========================================
  Files          72       72              
  Lines        3421     3429       +8     
  Branches     2120     2124       +4     
==========================================
  Hits         1933     1933              
+ Misses        572      562      -10     
- Partials      916      934      +18     

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