valid_url() for $_SERVER['HTTP_REFERER'] in simplesamlphp_auth_loginpage() causes crash of server for complex urls

Hi, I worked on this issue for days before that I finally figured out what was causing it.
Note also that I'm reporting this for version 7.x-3.x-dev but the same can occur on the other branch 7.x-2.x-dev.

As synthesized in the title, the check performed by the function valid_url() for $_SERVER['HTTP_REFERER'] in simplesamlphp_auth_loginpage() causes a severe crash of the server (Apache for Windows in my case) when the validation is performed on complex urls (e.g. https://idp-server.example.com/idp/?SAMLRequest=fZJBTwIxEIX%2Fyqb33e4uC0...).
This can be easily tested/reproduced with the "Execute PHP Code" functionality of Devel, trying to print the result of valid_url() for the url I wrote in the e.g. above.

My current solution was to remove only this check in the if clause, but probably this is not the best solution.