fixed some possible bugs


sql/item.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/item_subselect.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/item_sum.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/mysql_priv.h:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_base.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_cache.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_delete.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_lex.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_lex.h:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
  fixed some possible bugs
sql/sql_parse.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_prepare.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_repl.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_select.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_union.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_update.cc:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion
sql/sql_yacc.yy:
  removed SELECT_LEX_UNIT -> SELECT_LEX pointer conversion