Update purgePost() to validate page_for_posts

[jerclarke]

A small patch to fix a relatively-rare use case.

Our bug: Errors in logs when saving a post, caused by varnish-http-purge e.g:

PHP Notice: Undefined index: host in /SITE/wp-content/plugins/varnish-http-purge/varnish-http-purge.php on line 292

These were caused by purgeUrl() method being passed empty $url arguments. They CAN be solved by validating $url at that point:

public function purgeUrl($url) { if (!$url) return;

I won't submit a PR for that, but IMHO something like that would be valuable, especially if it left a log error with a backtrace. What follows took a lot of advanced debugging to figure out.

I wanted to identify the root cause of the empty $url, and traced it back through executePurge() (where the array had an empty item) to purgePost(), which added the empty item.

In the end it was the code that accounts for show_on_front that caused the problem for me. The reason is that it assumes that any site with page as get_option('show_on_front') will ALSO have a value for get_option('page_for_posts'). In my case this wasn't true, and the WP UI clearly lets this happen without any warnings (Screenshot: http://i.imgur.com/1uAWA2p.png )

This PR just adds a second validation of get_option('page_for_posts') to make sure it exists before adding get_permalink( get_option('page_for_posts') into the array of URLs to purge.

Thanks for merging this ASAP, and considering whether better validation of the URLs from the array would make sense in either purgeUrl() or executePurge().

P.S. Not sure what is up with the newline at end of file, tried to fix it sorry about that.