Comment 38 for bug 498933

I just had this bug on Ubuntu 17.10, duplicity 0.7.12

```
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1546, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1540, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1391, in main
    do_backup(action)
  File "/usr/bin/duplicity", line 1468, in do_backup
    restore(col_stats)
  File "/usr/bin/duplicity", line 731, in restore
    restore_get_patched_rop_iter(col_stats)):
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 560, in Write_ROPaths
    for ropath in rop_iter:
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 523, in integrate_patch_iters
    for patch_seq in collated:
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 389, in yield_tuples
    setrorps(overflow, elems)
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 378, in setrorps
    elems[i] = iter_list[i].next()
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 121, in difftar2path_iter
    tarinfo_list = [tar_iter.next()]
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 339, in next
    self.set_tarfile()
  File "/usr/lib/python2.7/dist-packages/duplicity/patchdir.py", line 333, in set_tarfile
    self.current_fp = self.fileobj_iter.next()
  File "/usr/bin/duplicity", line 767, in get_fileobj_iter
    backup_set.volume_name_dict[vol_num],
KeyError: 1

```