ringtest: pass buf != NULL

just a stub pointer for now.

Signed-off-by: Michael S. Tsirkin <[e-mail]>

tools/virtio/ringtest/main.c

···
 		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) {