Don't try to append "Display Name" options when on the Add New User screen...

Don't try to append "Display Name" options when on the Add New User screen (the <select> doesn't exist there), props pbiron, SergeyBiryukov, fixes #20392

git-svn-id: https://core.svn.wordpress.org/trunk@20407 1a063a9b-81f0-0310-95a4-ce76da25c4cd