Propel2 SQL injection possible with limit() on MySQL

Summary

The limit() query method is susceptible to catastrophic SQL injection with MySQL. For example, given a model User for a table users: ``` UserQuery::create()->limit('1;DROP TABLE users')->find(); ``` This will drop the users table! The cause appears to be a lack of integer casting of the limit input in either Propel\Runtime\ActiveQuery\Criteria::setLimit() or in Propel\Runtime\Adapter\Pdo\MysqlAdapter::applyLimit(). The code comments there seem to imply that casting was avoided due to overflow issues with 32-bit integers. This is surprising behavior since one of the primary purposes of an ORM is to prevent basic SQL injection. This affects all versions of Propel: 1.x, 2.x, and 3.