Add check for 'submit_page' entry in $edd_options


![screenshot_10_06_13_8_49_pm](https://f.cloud.github.com/assets/676582/631187/a61f92fa-d1bf-11e2-9bc0-5a23c6245a51.png)
On a brand new install, I found that this line triggered a PHP notice, because the submit_page variable had not yet been set. This additional check removes the notice.