[ticket/17413] Fix setting solved state on valid confirm code

PHPBB-17413

phpBB/phpbb/captcha/plugins/base.php

@@ -87,6 +87,7 @@ abstract class base implements plugin_interface
 	{
 		if ($this->confirm_id && hash_equals($this->code, $this->confirm_code))
 		{
+			$this->solved = true;
 			return true;
 		}