There is an optimization of DISTINCT in JOIN::optimize()
which depends on THD::used_tables value. Each SELECT statement
inside SP resets used_tables value(see mysql_select()) and it
leads to wrong result. The fix is to replace THD::used_tables
with LEX::used_tables.


mysql-test/r/sp.result:
  test case
mysql-test/t/sp.test:
  test case
sql/sql_base.cc:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_class.cc:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_class.h:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_insert.cc:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_lex.cc:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_lex.h:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_prepare.cc:
  THD::used_tables is replaced with LEX::used_tables
sql/sql_select.cc:
  THD::used_tables is replaced with LEX::used_tables