tar: do not crash with --listed-incremental

Problem reported by Frantisek Hanzlik in
<https://bugzilla.redhat.com/635318> via Kamil Dudka in
<http://lists.gnu.org/archive/html/bug-tar/2010-09/msg00066.html>.
I don't understand this code either, but Sergey can take a look at
this patch, and perhaps install a better one, when he has the time.
* src/incremen.c (append_incremental_renames): Don't actually append
anything to DIR if DIR is null.