Fix an integer type mismatch

storage/innobase/include/page0page.ic

 /*****************************************************************************
 
 Copyright (c) 1994, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2016, 2017, MariaDB Corporation.
+Copyright (c) 2016, 2018, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -192,13 +192,11 @@ page_header_get_offs(
 	const page_t*	page,	/*!< in: page */
 	ulint		field)	/*!< in: PAGE_FREE, ... */
 {
-	ulint	offs;
-
 	ut_ad((field == PAGE_FREE)
 	      || (field == PAGE_LAST_INSERT)
 	      || (field == PAGE_HEAP_TOP));
 
-	offs = page_header_get_field(page, field);
+	uint16_t offs = page_header_get_field(page, field);
 
 	ut_ad((field != PAGE_HEAP_TOP) || offs);