CRM-16832. Do not redirect offsite when fed invalid keys.
URLs to test:
* http://civicrm.dev/civicrm/contribute/transact?qfKey=xxx&entryURL=http://evil.example.com/ should go to CIVICRM_UF_BASEURL
* http://civicrm.dev/civicrm/contribute/transact?qfKey=xxx&entryURL=/civicrm/contribute/transact%3Fid%3D1 should go to /civicrm/contribute/transact?id=1
* http://civicrm.dev/civicrm/contribute/transact?qfKey=xxx&entryURL=http://civicrm.dev/civicrm/contribute/transact%3Fid%3D1 should go to /civicrm/contribute/transact?id=1