[ticket/12352] Do not check hashes that don't have the necessary length

This should significantly reduce the time spent on checking hashes of
passwords that should be converted.

PHPBB3-12352

phpBB/phpbb/passwords/driver/bcrypt_wcf2.php

@@ -65,7 +65,7 @@ class bcrypt_wcf2 extends base
 	*/
 	public function check($password, $hash, $user_row = array())
 	{
-		if (empty($hash))
+		if (empty($hash) || strlen($hash) != 60)
 		{
 			return false;
 		}