Bug#46949: memory leak with failed alter table to create partitions based on extract()

Bug#51830: Incorrect partition pruning on range partition (regression)

Fixed valgrind failure in select_describe(), read of uninitialized 
  Item_subselect::eliminated.

PBXT test file updates to reflect changes done in MySQL.

mysql-test/suite/pbxt/r/partition_error.result:
  Result file update following MySQL 5.1.44 changes.
mysql-test/suite/pbxt/r/partition_pruning.result:
  Result file update following MySQL 5.1.44 changes.
mysql-test/suite/pbxt/t/partition_error.test:
  Test file update following MySQL 5.1.44 changes.
sql/item_subselect.cc:
    Fixed valgrind failure in select_describe(), read of uninitialized 
    Item_subselect::eliminated: 
    - it turns out we can call select_describe() without having fixed 
      subquery items for child subselects. These are not the kind of subqueries
      that we could eliminate, so the fix is to ensure that 
      item_subselect->eliminated==FALSE even before fix_fields is called.
      Also added code to reset item_subselect->eliminated back to FALSE in
      Item::reset() call.
sql/item_subselect.h:
    Fixed valgrind failure in select_describe(), read of uninitialized 
    Item_subselect::eliminated: 
    - it turns out we can call select_describe() without having fixed 
      subquery items for child subselects. These are not the kind of subqueries
      that we could eliminate, so the fix is to ensure that 
      item_subselect->eliminated==FALSE even before fix_fields is called.
      Also added code to reset item_subselect->eliminated back to FALSE in
      Item::reset() call.
sql/sql_partition.cc:
  Fix Bug#51830: Revert part of the patch for Bug#49742, which caused the regression.
sql/table.cc:
  Fix Bug#46949: memory leak in failed ALTER TABLE with partitioning.