authX509: Fix cross-site scripting.

Fixes a potential cross-site scripting vulnerability on the error page.
Fortuitously, a different bug prevents us from ever reaching this page,
so this bug cannot be exploited.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2436 44740490-163a-0410-bde0-09ae8108e29a