xattr: fix assert that breaks importctl (#37146)

xattr: fix assert that breaks importctl (#37146)

getxattr_at_malloc() calls getxattr_pinned_internal() with size 0 to get
the size of the xattr, and then asserts because n > 0
```
Assertion '(size_t) n <= size' failed at src/basic/xattr-util.c:107, function getxattr_pinned_internal().
```

Follow-up for b8df25dcfe674e37ceb3d54e00a31e1d33e96057