Check if post exists before deleting

If you do a bulk delete and then go to the previous page using pagination you get a ton load of notices -

`Notice: Trying to get property of non-object in /wp-content/plugins/easy-digital-downloads/includes/payments/functions.php on line 254`

We should check if the post exists before attempting to delete it.
Merge pull request #2543 from elliott-stocks/patch-6

Check if post exists before deleting