Use correct URL in post_formats_compat()

[tollmanz]

When getting the URL for the link post format in post_formats_compat(), the incorrect URL value is used. Within the switch statement, when link is the case, the first conditional statement gets the URL that represents the link and assigns it to $url. Within the next conditional, the HTML for the link is composed. Instead of using $url it uses $meta['link_url']. This value may not be set, which will result in a blank URL being printed (e.g., <a href="http://google.com"></a>).

To reproduce the issue:

1. Add a new post of link post format
2. Do not add anything in the Title or Link URL fields
3. Type "​http://google.com" in the post content

This results in:

<a href="http://google.com"></a>

When the expected output is:

<a href="http://google.com">http://google.com</a>

​24419.patch uses $url for the text value of the link.

In 24353:

Use correct variable. props tollmanz. fixes #24419.