BUG#20310212 PARTITION DDL- CRASH AFTER THD::NOCHECK_REGISTER_ITEM_

Problem :
---------
Issue-1: The root cause for the issues is that (col1 > 1) is not a
valid partition function and we should have thrown error at much early
stage [partition_info::check_partition_info]. We are not checking
sub-partition expression when partition expression is NULL.

Issue-2: Potential issue for future if any partition function needs to
change item tree during open/fix_fields. We should release changed
items, if any, before doing closefrm when we open the partitioned table
during creation in create_table_impl.

Solution :
----------
1.check_partition_info() - Check for sub-partition expression even if no
partition expression.
[partition by ... columns(...) subpartition by hash(<expr>)]

2.create_table_impl() - Assert that the change list is empty before doing
closefrm for partitioned table. Currently no supported partition function
seems to be changing item tree during open.

Reviewed-by: Mattias Jonsson <mattias.jonsson@oracle.com>

RB: 9345

mysql-test/t/partition_error.test

@@ -1145,7 +1145,7 @@ subpartition by hash (rand(a+b));
 #
 # Subpartition by hash, wrong subpartition function
 #
---error ER_SUBPARTITION_ERROR
+--error ER_PARTITION_FUNCTION_IS_NOT_ALLOWED
 CREATE TABLE t1 (
 a int not null,
 b int not null,