No need to fill random bytes for VERIFY_NULL

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/verify.c b/verify.c
index f1ac43b..99b21e8 100644
--- a/verify.c
+++ b/verify.c
@@ -129,6 +129,9 @@
 	unsigned char *p = (unsigned char *) io_u->buf;
 	struct verify_header hdr;
 
+	if (td->o.verify == VERIFY_NULL)
+		return;
+
 	hdr.fio_magic = FIO_HDR_MAGIC;
 	hdr.len = io_u->buflen;
 	p += sizeof(hdr);