xattr: fix crash on dangling symlinks

Summary:
When requesting metadata on a dangling symlink, the framestack ends up calling
`k_queryAttributes()` with the symlink path, where the `listxattr` syscall
returns `size==-1` and `errno==ENOENT` (No such file or directory).
This case was not covered before, and provoked a segfault on `QByteArray`.
Full traceback on: https://bugs.kde.org/show_bug.cgi?id=414227

It might be also a good idea to always protect the function when `size==-1`

BUG: 414227

Test Plan:
- `bin/usermetadatawritertest` : added test
- On dolphin, with panel information open, hover over a dangling symlink

Reviewers: astippich, bruns

Reviewed By: bruns

Subscribers: bruns, kde-frameworks-devel, #baloo

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D25414