xattrs: Fix possible double-free in get_xattrs_impl

When we free `xattr_name`, we need to transfer ownership away to avoid
potential for a double-free if we hit `ENOTSUP` on the next iteration.

Reported-by: Seth Arnold <seth.arnold@canonical.com>

glnx-xattrs.c

@@ -181,7 +181,7 @@ get_xattrs_impl (const char      *path,
         {
           if (errno == ERANGE)
             {
-              g_free (xattr_names);
+              g_free (g_steal_pointer (&xattr_names));
               goto again;
             }
           glnx_set_prefix_error_from_errno (error, "%s", "llistxattr");