Bug #31893 Partitions: crash if subpartitions and engine change.

The new default database engine for altered table was reassigned to
the old one. That's wrong thing by itself, and (as the engine
for a subpartition gets that new value) leads to DBUG_ASSERTION
in mysql_unpack_partition()

mysql-test/r/partition.result

@@ -1291,4 +1291,18 @@ t1	CREATE TABLE `t1` (
   `b` int(11) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 /*!50100 PARTITION BY RANGE (b) (PARTITION p1 VALUES LESS THAN (10) ENGINE = MyISAM, PARTITION p2 VALUES LESS THAN (20) ENGINE = MyISAM) */
 drop table t1, t2;
+create table t1 (int_column int, char_column char(5))
+PARTITION BY RANGE (int_column) subpartition by key (char_column)
+(PARTITION p1 VALUES LESS THAN (5) ENGINE = InnoDB);
+Warnings:
+Warning	1286	Unknown table engine 'InnoDB'
+alter table t1 PARTITION BY RANGE (int_column) subpartition by key (char_column)
+(PARTITION p1 VALUES LESS THAN (5) ENGINE = myisam);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `int_column` int(11) DEFAULT NULL,
+  `char_column` char(5) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 /*!50100 PARTITION BY RANGE (int_column) SUBPARTITION BY KEY (char_column) (PARTITION p1 VALUES LESS THAN (5) ENGINE = MyISAM) */
+drop table t1;
 End of 5.1 tests

mysql-test/t/partition.test

@@ -1528,4 +1528,15 @@ PARTITION BY RANGE (b) (
 show create table t1;
 drop table t1, t2;
 
+#
+# Bug #31893 Partitions: crash if subpartitions and engine change
+#
+create table t1 (int_column int, char_column char(5))
+  PARTITION BY RANGE (int_column) subpartition by key (char_column)
+  (PARTITION p1 VALUES LESS THAN (5) ENGINE = InnoDB);
+alter table t1 PARTITION BY RANGE (int_column) subpartition by key (char_column)
+  (PARTITION p1 VALUES LESS THAN (5) ENGINE = myisam);
+show create table t1;
+drop table t1;
+
 --echo End of 5.1 tests

sql/sql_partition.cc

@@ -5031,7 +5031,10 @@ the generated partition syntax in a correct manner.
         *partition_changed= TRUE;
       }
       if (create_info->db_type == partition_hton)
-        part_info->default_engine_type= table->part_info->default_engine_type;
+      {
+        if (!part_info->default_engine_type)
+          part_info->default_engine_type= table->part_info->default_engine_type;
+      }
       else
         part_info->default_engine_type= create_info->db_type;
       if (check_native_partitioned(create_info, &is_native_partitioned,