BUG#19067: Crash for default subpartitioning when altering the table

mysql-test/r/information_schema_part.result:
  Bug fix
sql/partition_element.h:
  Review fixes
sql/sql_partition.cc:
  Review fixes

mysql-test/r/information_schema_part.result

@@ -119,7 +119,7 @@ SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
   `a` int(11) DEFAULT NULL
-) ENGINE=MyISAM DEFAULT CHARSET=latin1 PARTITION BY RANGE (a) SUBPARTITION BY LINEAR HASH (a) (PARTITION p0 VALUES LESS THAN (10) )
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 PARTITION BY RANGE (a) SUBPARTITION BY LINEAR HASH (a) (PARTITION p0 VALUES LESS THAN (10) ENGINE = MyISAM)
 select SUBPARTITION_METHOD FROM information_schema.partitions WHERE
 table_schema="test" AND table_name="t1";
 SUBPARTITION_METHOD

sql/partition_element.h

@@ -64,24 +64,20 @@ public:
     engine_type(NULL),part_state(PART_NORMAL),
     nodegroup_id(UNDEF_NODEGROUP), has_null_value(FALSE)
   {
-    subpartitions.empty();
-    list_val_list.empty();
   }
   partition_element(partition_element *part_elem)
-  : partition_name(NULL), range_value(0), has_null_value(FALSE)
+  : part_max_rows(part_elem->part_max_rows),
+    part_min_rows(part_elem->part_min_rows),
+    partition_name(NULL),
+    tablespace_name(part_elem->tablespace_name),
+    range_value(0), part_comment(part_elem->part_comment),
+    data_file_name(part_elem->data_file_name),
+    index_file_name(part_elem->index_file_name),
+    engine_type(part_elem->engine_type),
+    part_state(part_elem->part_state),
+    nodegroup_id(part_elem->nodegroup_id),
+    has_null_value(FALSE)
   {
-    subpartitions.empty();
-    list_val_list.empty();
-
-    part_max_rows= part_elem->part_max_rows;
-    part_min_rows= part_elem->part_min_rows;
-    tablespace_name= part_elem->tablespace_name;
-    part_comment= part_elem->part_comment;
-    data_file_name= part_elem->data_file_name;
-    index_file_name= part_elem->index_file_name;
-    engine_type= part_elem->engine_type;
-    part_state= part_elem->part_state;
-    nodegroup_id= part_elem->nodegroup_id;
   }
   ~partition_element() {}
 };

sql/sql_partition.cc

@@ -1886,9 +1886,10 @@ char *generate_partition_syntax(partition_info *part_info,
         err+= add_partition_values(fptr, part_info, part_elem);
         if (!part_info->is_sub_partitioned() ||
             part_info->use_default_subpartitions)
+        {
           err+= add_partition_options(fptr, part_elem);
-        if (part_info->is_sub_partitioned() &&
-            (!part_info->use_default_subpartitions))
+        }
+        else
         {
           err+= add_space(fptr);
           err+= add_begin_parenthesis(fptr);