Fatal error on php5.4 with function-calls in empty-call

[idittmar]

Hi!

I got the following fatal error on our servers running on php 5.4.45:

Fatal error: Can't use method return value in write context in (...)/src/OpenIDConnectClient.php on line 664

Php crashing on that line by the following statement:
if (!empty($this->getCodeChallengeMethod()) ...

As this solution suggests, putting the result of inner getCodeChallengeMethod-call before empty-call into an extra variable might be a quickfix for that problem.

Same error should also be fixed at line 775

I already forked master, and commited the fix. But since i'm new to github, i'm a bit unsure how to transfer it back into your repo?