on re-execution of prepared statement

Problem: some (see eq_ref_table()) ORDER BY/GROUP BY optimization
is called before each PS execution. However, we don't properly 
initialize its stucture every time before the call.

Fix: properly initialize the sturture used.



mysql-test/r/ps.result:
  Fix for bug#49570: Assertion failed: !(order->used & map) 
  on re-execution of prepared statement
    - test result.
mysql-test/t/ps.test:
  Fix for bug#49570: Assertion failed: !(order->used & map) 
  on re-execution of prepared statement
    - test case.
sql/sql_select.cc:
  Fix for bug#49570: Assertion failed: !(order->used & map) 
  on re-execution of prepared statement
    - set order->used to 0 before each eq_ref_table() call,
  as the function relies on that.