Default verify backlog batch to verify backlog setting if not given

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/io_u.c b/io_u.c
index 76ad933..bffacb4 100644
--- a/io_u.c
+++ b/io_u.c
@@ -935,6 +935,8 @@
 		} else if (!(td->io_hist_len % td->o.verify_backlog) &&
 			 td->last_ddir != DDIR_READ) {
 			td->verify_batch = td->o.verify_batch;
+			if (!td->verify_batch)
+				td->verify_batch = td->o.verify_backlog;
 			get_verify = 1;
 		}