Fix FreeBSD warning

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/os/os-freebsd.h b/os/os-freebsd.h
index 3115b60..bb7e9a3 100644
--- a/os/os-freebsd.h
+++ b/os/os-freebsd.h
@@ -22,6 +22,7 @@
 		return 0;
 	}
 
+	*bytes = 0;
 	return errno;
 }