- Corrected problem with N-way nested natural joins in PS mode.
- Code cleanup
- More asserts to check consistency of name resolution contexts
- Fixed potential memory leak of name resolution contexts


mysql-test/r/join.result:
  - Corrected problem with N-way nested natural joins in PS mode.
mysql-test/t/join.test:
  - Corrected problem with N-way nested natural joins in PS mode.
sql/item.h:
  - Fixed potential memory leak.
sql/sql_base.cc:
  - the local context of Item_fields that participate in TABLE_LIST::on_cond for
    natural joins is correctly set to the tables where the corresponding fields
    originate from.
  - removed unused variables
  - correct allocation of contexts
sql/sql_parse.cc:
  - correct allocation of contexts for JOIN ON conditions.
sql/table.cc:
  - added asserts to check the consistency of name resolution contexts
sql/table.h:
  - added asserts to check the consistency of name resolution contexts