Do not reset value of LAST_INSERT_ID() in sub-statement.


mysql-test/r/rpl_insert_id.result:
  Add result for bug#15728.
mysql-test/r/sp.result:
  Add result for bug#15728.
mysql-test/t/rpl_insert_id.test:
  Add test case for bug#15728.
mysql-test/t/sp.test:
  Add test case for bug#15728.
sql/sql_class.cc:
  Do not reset value of LAST_INSERT_ID() in sub-statement.