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>

block/iscsi.c

@@ -753,7 +753,7 @@ retry:
     }
 
     lbas = scsi_datain_unmarshall(iTask.task);
-    if (lbas == NULL) {
+    if (lbas == NULL || lbas->num_descriptors == 0) {
         ret = -EIO;
         goto out_unlock;
     }