Ensure post ID evaluates to true before adding "Edit post" tab in admin bar

[Joe Anzalone]

This patch checks that a post has an ID set before adding an "Edit post" link in the admin bar on the front-end for individual post pages and static pages.

This can be helpful for plugins that choose to create a "fake" page that does not exist in the database, and set their ID to "0."

An example of such a fake page can be seen on my ​On The Fly YouTube Embeds plugin which borrows a lot of code from Scott Sherrill-Mix's blog post: ​Creating a (Better) Fake Post with a WordPress Plugin

[morganestes]

I've tested this out and it works in 4.3.0 trunk. Since we're using $current_object->ID as a parameter twice in the same if check, it makes sense to check it for a valid value beforehand.
That also takes care of the example @Joe Anzalone provided with plugins creating their own posts with an ID of 0.

[morganestes]

Related: #33154

[akibjorklund]

Not sure what the reported issue actually is, but if I understand correctly, the ticket is about an edit link appearing when there should be none. If this is true, it does not seem to be an issue anymore. get_edit_post_link will fetch the post using get_post and will not return a valid link if the post is not found. I've attached a patch that contains unit tests to prove this.

While the issue itself has probably been fixed within some other ticket and the original patch is unnecessary, the unit tests I created could still be committed.

[jorbin]

In 38508:

Toolbar: Add unit tests for edit links.

When there is no post ID, there should be no edit link. This adds unit tests for it.

Fixes #22247.
Props akibjorklund.