In `get_the_author_posts()`, if there is no current `$post`, return `0` and bail.

Props krogsgard, aaroncampbell.
Fixes #27998.


Built from https://develop.svn.wordpress.org/trunk@28362


git-svn-id: http://core.svn.wordpress.org/trunk@28190 1a063a9b-81f0-0310-95a4-ce76da25c4cd