ringtest: pass buf != NULL

just a stub pointer for now.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/virtio/ringtest/main.c b/tools/virtio/ringtest/main.c
index 3a5ff43..147abb4 100644
--- a/tools/virtio/ringtest/main.c
+++ b/tools/virtio/ringtest/main.c
@@ -115,7 +115,7 @@
 		do {
 			if (started < bufs &&
 			    started - completed < max_outstanding) {
-				r = add_inbuf(0, NULL, "Hello, world!");
+				r = add_inbuf(0, "Buffer\n", "Hello, world!");
 				if (__builtin_expect(r == 0, true)) {
 					++started;
 					if (!--tokick) {