MDEV-13476 TRX_UNDO_PAGE_TYPE mismatch when writing undo log after upgrade

When undo log pages pre-exist from an upgrade, the
TRX_UNDO_PAGE_TYPE could be TRX_UNDO_INSERT==1 (for insert_undo)
TRX_UNDO_UPDATE==2 (for update_undo), instead of the new unified
page type 0 that was introduced in MDEV-12288.

The undo log page type does not really matter much, because the
undo log record type identifies the records independently
of the page type. So, the debug assertions can simply allow any
potential value of the TRX_UNDO_PAGE_TYPE (0, 1, or 2).

storage/innobase/trx/trx0rec.cc

@@ -469,8 +469,12 @@ trx_undo_page_report_insert(
 	ulint		i;
 
 	ut_ad(dict_index_is_clust(index));
-	ut_ad(*reinterpret_cast<uint16*>(TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_TYPE
-					 + undo_page) == 0);
+	/* MariaDB 10.3.1+ in trx_undo_page_init() always initializes
+	TRX_UNDO_PAGE_TYPE as 0, but previous versions wrote
+	TRX_UNDO_INSERT == 1 into insert_undo pages,
+	or TRX_UNDO_UPDATE == 2 into update_undo pages. */
+	ut_ad(mach_read_from_2(TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_TYPE
+			       + undo_page) <= 2);
 
 	first_free = mach_read_from_2(undo_page + TRX_UNDO_PAGE_HDR
 				      + TRX_UNDO_PAGE_FREE);
@@ -875,8 +879,13 @@ trx_undo_page_report_modify(
 
 	ut_a(dict_index_is_clust(index));
 	ut_ad(rec_offs_validate(rec, index, offsets));
-	ut_ad(*reinterpret_cast<uint16*>(TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_TYPE
-					 + undo_page) == 0);
+	/* MariaDB 10.3.1+ in trx_undo_page_init() always initializes
+	TRX_UNDO_PAGE_TYPE as 0, but previous versions wrote
+	TRX_UNDO_INSERT == 1 into insert_undo pages,
+	or TRX_UNDO_UPDATE == 2 into update_undo pages. */
+	ut_ad(mach_read_from_2(TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_TYPE
+			       + undo_page) <= 2);
+
 	trx_undo_t*	undo = dict_table_is_temporary(table)
 		? NULL : trx->rsegs.m_redo.undo;