"ALTER SERVER can cause server to crash"
  While retrieving values, it would erronously set the socket value
  to NULL and attempt to use it in strcmp().
  Ensure it is correctly set to "" so that strcmp may not crash.


mysql-test/r/federated_server.result:
  results for bug30671
  fix inconsistent result
mysql-test/t/federated_server.test:
  surplus semicolon
  test for bug30671
sql/sql_servers.cc:
  bug30671
  inside function get_server_from_table_to_cache()
    server->socket was being set to NULL instead of empty string