Detect unsolvable volume cycle in split_bsr_loop()

Detect unsolvable volume cycle in split_bsr_loop()

- Fix empty part: when a cycle inside a job was detected, the function
  was creating an empty part (splitting at offset 0), that was causing
  a fatal error in the restore
- this make that the first part to restore is totally empty and
  generate the messages from the SD and from the FD:
  SD - Fatal error: No Volume names found for restore.
  FD - Fatal error: job.c:3694 Bad response from SD to Read Data command. Wanted 3000 OK data

- in the trace file we have a split at off=0
  BSR: Split the BSR at off=0 at the beginning of jobnum=1 ....

Signed-off-by: Alain Spineux <alain@baculasystems.com>