MDEV-31775 Server crash upon online alter on sequence

Forbid online for sequences. It doesn't work now and sequence tables are
always only one row, so not worth the extra complexity.

mysql-test/main/alter_table_online.result

@@ -219,3 +219,8 @@ create temporary table t (f int);
 alter table t force, algorithm=instant;
 ERROR 0A000: ALGORITHM=INSTANT is not supported for this operation. Try ALGORITHM=COPY
 drop table t;
+create sequence s engine=MyISAM;
+alter table s engine=Aria, lock=none;
+ERROR 0A000: LOCK=NONE is not supported. Reason: SEQUENCE. Try LOCK=SHARED
+alter table s engine=Aria;
+drop sequence  s;

mysql-test/main/alter_table_online.test

@@ -222,3 +222,9 @@ create temporary table t (f int);
 --error ER_ALTER_OPERATION_NOT_SUPPORTED
 alter table t force, algorithm=instant;
 drop table t;
+
+create sequence s engine=MyISAM;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table s engine=Aria, lock=none;
+alter table s engine=Aria;
+drop sequence  s;

sql/sql_table.cc

@@ -9916,6 +9916,10 @@ const char *online_alter_check_supported(const THD *thd,
   if (!*online)
     return NULL;
 
+  *online= table->s->sequence == NULL;
+  if (!*online)
+    return "SEQUENCE";
+
   *online= (alter_info->flags & ALTER_DROP_SYSTEM_VERSIONING) == 0;
   if (!*online)
     return "DROP SYSTEM VERSIONING";