Correct listfattr() error handling.
Correct listfattr() error handling.

It is necessary to determine between states of
 * no extended attributes found on the file (resulting in empty array being
   returned) and
 * error, i.e. ‘undef’ returned (resulting in an array with exactly one ‘undef’
   element).

michal-ruzicka/FSTreeIntegrityWatch,michal-ruzicka/FSTreeIntegrityWatch