We should disallow usage of RETURN statement in triggers and emit
error at parsing time (instead of crashing when trigger is fired).


mysql-test/r/trigger.result:
  Added test for bug #16829 "Firing trigger with RETURN crashes the server".
mysql-test/t/trigger.test:
  Added test for bug #16829 "Firing trigger with RETURN crashes the server".
sql/sql_yacc.yy:
  We should disallow usage of RETURN statement in triggers and emit
  error at parsing time (instead of crashing when trigger is fired).