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
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Do you need an issue raised for this as well? I hope not :-)
Merge pull request #2544 from jonjennings/patch-3

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Allow the list of registered settings to be filtered through "edd_registered_settings"