Properly redirect to install.php when visiting wp-login.php with a config file...

Properly redirect to install.php when visiting wp-login.php with a config file but no db. Props SergeyBiryukov. fixes #18397

git-svn-id: https://core.svn.wordpress.org/trunk@19923 1a063a9b-81f0-0310-95a4-ce76da25c4cd