easy/xattr: Return proper error code when malloc fails

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@898bf37). Learn more about missing BASE report.

Files	Patch %	Lines
libsqsh/src/easy/xattr.c	0.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #273 +/- ## ======================================= Coverage ? 64.76% ======================================= Files ? 74 Lines ? 5535 Branches ? 830 ======================================= Hits ? 3585 Misses ? 1471 Partials ? 479 ```

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