iscsi: Don't access non-existent scsi_lba_status_descriptor

iscsi: Don't access non-existent scsi_lba_status_descriptor

In iscsi_co_block_status(), we may have received num_descriptors == 0
from the iscsi server. Therefore, we can't unconditionally access
lbas->descriptors[0]. Add the missing check.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Felipe Franciosi <felipe@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Peter Lieven <pl@kamp.de>