Do not demand that buf_page_t be fully initialized on 64-bit systems.

There may be padding before buf_page_t::zip. (Bug #53307)

storage/innodb_plugin/buf/buf0lru.c

@@ -1393,7 +1393,12 @@ buf_LRU_free_block(
 	ut_ad(buf_page_in_file(bpage));
 	ut_ad(bpage->in_LRU_list);
 	ut_ad(!bpage->in_flush_list == !bpage->oldest_modification);
+#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(bpage, sizeof *bpage);
+#endif
 
 	if (!buf_page_can_relocate(bpage)) {
 
@@ -1688,7 +1693,12 @@ buf_LRU_block_remove_hashed_page(
 	ut_a(buf_page_get_io_fix(bpage) == BUF_IO_NONE);
 	ut_a(bpage->buf_fix_count == 0);
 
+#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(bpage, sizeof *bpage);
+#endif
 
 	buf_LRU_remove_block(bpage);
 

storage/innodb_plugin/include/buf0buf.ic

@@ -931,7 +931,12 @@ buf_page_hash_get(
 		ut_a(buf_page_in_file(bpage));
 		ut_ad(bpage->in_page_hash);
 		ut_ad(!bpage->in_zip_hash);
+#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(bpage, sizeof *bpage);
+#endif
 	}
 
 	return(bpage);