UR-4833: Fix membership not assigned and auto login not working

Fire user_registration_validate_form_data with do_action_ref_array() instead of
apply_filters_ref_array().

The hook passes &self::$valid_form_data by reference and its return value is
discarded, so it is an action in all but name. WP_Hook::apply_filters() does
$args[0] = $value before invoking each callback, and because $args[0] is a
reference to self::$valid_form_data, that assignment writes through the
reference. UR_Form_Validation::validate_form() is registered with add_action()
and has no return statement, so as soon as a second callback is attached to the
hook, $valid_form_data is overwritten with null and every submitted field is lost.

Consequences: user_login became empty and registration failed with
empty_user_login, and the membership field was gone so
process_membership_after_registration() silently skipped enrollment, leaving an
account with no plan, no order and no subscription. do_action_ref_array() sets
doing_action, which skips the $args[0] assignment, so nothing is lost.

Author: saurab018

includes/frontend/class-ur-frontend-form-handler.php

@@ -99,7 +99,7 @@ public static function handle_form( $form_data, $form_id ) {
 		 *
 		 * @return array
 		 */
-		apply_filters_ref_array(
+		do_action_ref_array(
 			'user_registration_validate_form_data',
 			array(
 				&self::$valid_form_data,