Post fix for bug#23800.
  The Item_field constructor now increases the select_n_where_fields counter.
sql_yacc.yy:
  Post fix for bug#23800.
  Take into account fields that might be added by subselects.
sql_lex.h:
  Post fix for bug#23800.
  Added the select_n_where_fields variable to the st_select_lex class.
sql_lex.cc:
  Post fix for bug#23800.
  Initialization of the select_n_where_fields variable.