Silence a spurious valrgind warning:

==2229== Uninitialised byte(s) found during client check request
==2229==    at 0x9A599D: buf_page_get_gen (buf0buf.c:3003)
==2229==    by 0x9D58CD: fsp_header_init (fsp0fsp.c:974)
==2229==    by 0x95BEAE: innobase_start_or_create_for_mysql (srv0start.c:1537)
==2229==    by 0x91DDBB: innobase_init(void*) (ha_innodb.cc:2409)
==2229==    by 0x73AF1F: ha_initialize_handlerton(st_plugin_int*) (handler.cc:475)
==2229==    by 0x5DBAE3: plugin_initialize(st_plugin_int*) (sql_plugin.cc:1057)
==2229==    by 0x5DD185: plugin_init(int*, char**, int) (sql_plugin.cc:1343)
==2229==    by 0x53830A: init_server_components() (mysqld.cc:4141)
==2229==    by 0x539048: mysqld_main(int, char**) (mysqld.cc:4742)
==2229==    by 0x52D412: main (main.cc:24)
==2229==  Address 0x998B00C is not stack'd, malloc'd or (recently) free'd
==2229== (Memcheck does not allow error to be suppressed)

Approved by:	Marko (rb://345)

storage/innobase/buf/buf0buf.c

@@ -3000,7 +3000,12 @@ wait_until_unfixed:
 	ut_ad(buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);
 
 	mutex_enter(&block->mutex);
+#if UNIV_WORD_SIZE == 4
+	/* On 32-bit systems, there is no padding in buf_page_t.  On
+	other systems, Valgrind could complain about uninitialized pad
+	bytes. */
 	UNIV_MEM_ASSERT_RW(&block->page, sizeof block->page);
+#endif
 
 	buf_block_buf_fix_inc(block, file, line);