wpdb: set dbh to null when the mysqli connection fails. see #21663.


git-svn-id: https://develop.svn.wordpress.org/trunk@27277 602fd350-edb4-49c9-b593-d223f7449a82
Add a constant to disable mysqli for testing purposes. see #21663.


git-svn-id: https://develop.svn.wordpress.org/trunk@27278 602fd350-edb4-49c9-b593-d223f7449a82