Don't set the site_name cache on every call to get_current_site_name(). Props...

Don't set the site_name cache on every call to get_current_site_name().  Props mark-k, solarissmoke. fixes #16676

git-svn-id: https://core.svn.wordpress.org/trunk@18604 1a063a9b-81f0-0310-95a4-ce76da25c4cd