Fix Valgrind build after e7924a8

Author: dr-m

storage/innobase/buf/buf0flu.cc

@@ -431,7 +431,7 @@ buf_flush_insert_into_flush_list(
 	ut_d(block->page.in_flush_list = TRUE);
 	ut_ad(!block->page.oldest_modification);
 	block->page.oldest_modification = lsn;
-	UNIV_MEM_ASSERT_RW(block->page.zip
+	UNIV_MEM_ASSERT_RW(block->page.zip.data
 			   ? block->page.zip.data : block->frame,
 			   block->page.size.physical());
 	incr_flush_list_size_in_bytes(block, buf_pool);