iscsi: Don't set error if already set in iscsi_do_inquiry

This eliminates the possible assertion failure in error_setg().
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/iscsi.c

@@ -1101,8 +1101,10 @@ static struct scsi_task *iscsi_do_inquiry(struct iscsi_context *iscsi, int lun,
     return task;
 
 fail:
+    if (!error_is_set(errp)) {
         error_setg(errp, "iSCSI: Inquiry command failed : %s",
                    iscsi_get_error(iscsi));
+    }
     if (task != NULL) {
         scsi_free_scsi_task(task);
     }