Cleanup error handling in ha_federatedx

1. Class ha_federatedx_derived_handler overrides virtual function derived_handler::print_error() with an empty implementation which causes problems with error handling. Errors are being simply ignored and occur during later processing.

void ha_federatedx_derived_handler::print_error(int, unsigned long)

{

}

2. Members of federatedx_handler_base are accessed in the end_scan_() function without checking for having been initialized. This can cause server crash if init_scan_() has not completed successfully.

int federatedx_handler_base::end_scan_()

{

  DBUG_ENTER("ha_federatedx_derived_handler::end_scan");

  (*iop)->free_result(stored_result);

  free_share(txn, share);

  DBUG_RETURN(0);

}