examples/vhost_blk: check unused value on init

Add the assert to handle error.

Coverity issue: 350592
Fixes: c19beb3f38 ("examples/vhost_blk: introduce vhost storage sample")
Cc: stable@dpdk.org

Signed-off-by: Jin Yu <jin.yu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

examples/vhost_blk/vhost_blk.c

@@ -856,6 +856,7 @@ new_device(int vid)
 				ctrlr->bdev->vid, i,
 				&blk_vq->last_avail_idx,
 				&blk_vq->last_used_idx);
+			assert(ret == 0);
 
 			blk_vq->avail_wrap_counter = blk_vq->last_avail_idx &
 				(1 << 15);