MDEV-13020 Server crashes in Item_func_nextval::val_int...

The problem was that we didn't check on open of sequence if the table is a view, which is not allowed.
We are now generating a proper error message for this case.

mysql-test/suite/sql_sequence/view.result

+create sequence s1;
+create view v1 as select * from s1;
+create view v2 as select next value for s1;
+select * from v1;
+next_not_cached_value	minimum_value	maximum_value	start_value	increment	cache_size	cycle_option	cycle_count
+1	1	9223372036854775806	1	1	1000	0	0
+select * from v2;
+next value for s1
+1
+select * from v2;
+next value for s1
+2
+select next value for v1;
+ERROR 42S02: 'test.v1' is not a SEQUENCE
+drop sequence s1;
+drop view v1,v2;
+#
+# MDEV 13020 Server crashes in Item_func_nextval::val_int upon
+# selecting NEXT or PREVIOUS VALUE for a view
+#
+CREATE OR REPLACE VIEW v1 AS SELECT 1 AS f;
+SELECT NEXT VALUE FOR v1;
+ERROR 42S02: 'test.v1' is not a SEQUENCE
+SELECT PREVIOUS VALUE FOR v1;
+ERROR 42S02: 'test.v1' is not a SEQUENCE
+drop view v1;

mysql-test/suite/sql_sequence/view.test

+--source include/have_sequence.inc
+--source include/have_innodb.inc
+
+#
+# Test sequences with views
+#
+
+create sequence s1;
+create view v1 as select * from s1;
+create view v2 as select next value for s1;
+select * from v1;
+select * from v2;
+select * from v2;
+--error ER_NOT_SEQUENCE
+select next value for v1;
+drop sequence s1;
+drop view v1,v2;
+
+--echo #
+--echo # MDEV 13020 Server crashes in Item_func_nextval::val_int upon
+--echo # selecting NEXT or PREVIOUS VALUE for a view
+--echo #
+
+CREATE OR REPLACE VIEW v1 AS SELECT 1 AS f;
+--error ER_NOT_SEQUENCE
+SELECT NEXT VALUE FOR v1;
+--error ER_NOT_SEQUENCE
+SELECT PREVIOUS VALUE FOR v1;
+drop view v1;

sql/sql_base.cc

@@ -1783,7 +1783,11 @@ bool open_table(THD *thd, TABLE_LIST *table_list, Open_table_context *ot_ctx)
       my_error(ER_WRONG_MRG_TABLE, MYF(0));
       goto err_lock;
     }
-
+    if (table_list->sequence)
+    {
+      my_error(ER_NOT_SEQUENCE, MYF(0), table_list->db, table_list->alias);
+      goto err_lock;
+    }
     /*
       This table is a view. Validate its metadata version: in particular,
       that it was a view when the statement was prepared.