MDEV-13937 Aria engine: Internal Error 160 after partition handling

Partition wasn't setting HA_OPTION_PACK_RECORD on ALTER TABLE
if the row format was PAGE.

(so one bit in the null bitmap was reserved for a deleted bit -
see make_empty_rec - and all actual null bits were one off)

mysql-test/suite/parts/r/partition_alter_maria.result

+create table t1 (
+pk bigint not null auto_increment,
+dt datetime default null,
+unique (pk, dt)
+) engine=aria row_format=dynamic
+partition by range  columns(dt) (
+partition `p20171231` values less than ('2017-12-31'),
+partition `p20181231` values less than ('2018-12-31')
+);
+insert into t1 values (1,'2017-09-28 15:12:00');
+select * from t1;
+pk	dt
+1	2017-09-28 15:12:00
+alter table t1 drop partition p20181231;
+select * from t1;
+pk	dt
+1	2017-09-28 15:12:00
+drop table t1;

mysql-test/suite/parts/t/partition_alter_maria.test

+#
+# MDEV-13937 Aria engine: Internal Error 160 after partition handling 
+#
+source include/have_partition.inc;
+create table t1 (
+  pk bigint not null auto_increment,
+  dt datetime default null,
+  unique (pk, dt)
+) engine=aria row_format=dynamic
+  partition by range  columns(dt) (
+    partition `p20171231` values less than ('2017-12-31'),
+    partition `p20181231` values less than ('2018-12-31')
+);
+insert into t1 values (1,'2017-09-28 15:12:00');
+select * from t1;
+alter table t1 drop partition p20181231;
+select * from t1;
+drop table t1;

sql/sql_partition.cc

@@ -6672,7 +6672,8 @@ uint fast_alter_partition_table(THD *thd, TABLE *table,
   lpt->alter_info= alter_info;
   lpt->create_info= create_info;
   lpt->db_options= create_info->table_options;
-  if (create_info->row_type == ROW_TYPE_DYNAMIC)
+  if (create_info->row_type != ROW_TYPE_FIXED &&
+      create_info->row_type != ROW_TYPE_DEFAULT)
     lpt->db_options|= HA_OPTION_PACK_RECORD;
   lpt->table= fast_alter_table;
   lpt->old_table= table;