Improve robustness of the code that restores .queue_pdu()

In my tests I hit a stack overflow caused by infinite recursion of
.queue_pdu() calls, caused by .queue_pdu() not being restored in an
error path. Make the approach for restoring .queue_pdu() more robust.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

test-tool/iscsi-test-cu.c

@@ -730,6 +730,7 @@ static struct test_family families[] = {
  */
 struct scsi_task *task;
 unsigned char *read_write_buf;
+int (*orig_queue_pdu)(struct iscsi_context *iscsi, struct iscsi_pdu *pdu);
 
 static void
 print_usage(void)
@@ -825,11 +826,14 @@ test_setup(void)
 {
         task = NULL;
         read_write_buf = NULL;
+        orig_queue_pdu = sd->iscsi_ctx ? sd->iscsi_ctx->drv->queue_pdu : NULL;
 }
 
 void
 test_teardown(void)
 {
+        if (sd->iscsi_ctx)
+                sd->iscsi_ctx->drv->queue_pdu = orig_queue_pdu;
         free(read_write_buf);
         read_write_buf = NULL;
         scsi_free_scsi_task(task);