btrfs-progs: tests: check nullb index for the error case

"_find_free_index" can return "" when an error occur. Check the return
value for the case and fail the test.

Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Naohiro Aota <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: naota

tests/nullb

@@ -146,6 +146,9 @@ fi
 if [ "$CMD" = 'create' ]; then
 	_check_setup
 	index=$(_find_free_index)
+	if [ -z "$index" ]; then
+		exit 1
+	fi
 	name="nullb$index"
 	# size in MB
 	size=$(_parse_device_size "$@")