test: fix nbd test typo and add check

Change-Id: I5120baff8d2de3ef639d7b3c66d51002838c3f3c
Signed-off-by: Liu Xiaodong <xiaodong.liu@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8439
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

test/bdev/blockdev.sh

@@ -153,7 +153,11 @@ function nbd_function_test() {
 		local conf=$1
 		local nbd_all=($(ls /dev/nbd* | grep -v p))
 		local bdev_all=($bdevs_name)
-		local nbd_num=${#bdevs_all[@]}
+		local nbd_num=${#bdev_all[@]}
+		if ((nbd_num < 1)); then
+			# There should be at least one bdev and one valid nbd device
+			return 1
+		fi
 		if [ ${#nbd_all[@]} -le $nbd_num ]; then
 			nbd_num=${#nbd_all[@]}
 		fi