bbPress: Ensure 'reply_to' variable uses bbp_get_reply_to().

Fixes issues with setting the reply to ID when nested replies is on in
bbPress.

See commit 606b526.

includes/bp-rbe-extend-bbpress.php

@@ -1365,7 +1365,7 @@ public function ges_extend_listener( $rbe ) {
 		// If activity type is a bbPress reply, we need to grab the topic ID manually.
 		if ( $this->temp_activity->type === $this->activity_type ) {
 			$rbe->listener->secondary_item_id = bbp_get_reply_topic_id( $this->temp_activity->secondary_item_id );
-			$rbe->listener->reply_to_id       = get_post_field( 'post_parent', $this->temp_activity->secondary_item_id );
+			$rbe->listener->reply_to_id       = bbp_get_reply_to( $this->temp_activity->secondary_item_id );
 		}
 	}
 }