Fix #7286 DIR segfault when doing a "dir" command in a restore

Fix #7286 DIR segfault when doing a "dir" command in a restore

- this initialize "ua->jcr->db" as it is done few lines below at the first
  initialization
- the ticket shows that ua->db was used successfully by other functions
  few line before the crash while ua->jcr->db == NULL.
- it could be a fix (I cannot shows that)
- it could be a new bug (I don't see how)
- it is probably an improvement as the first and the next calls will
  return a consistent "ua"