metaedit: Fix cross-site scripting.

metaedit fails to validate the userid. If a malicious user is able to
make another user log in as that user id, he will be able to run scripts
in the domain of the site.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2442 44740490-163a-0410-bde0-09ae8108e29a