Update OpenIDConnectClient.php5

Eliminate warning when $this->redirectURL has not previously been set:

PHP Notice:  Undefined property: OpenIDConnectClient::$redirectURL in .../OpenIDConnectClient.php5 on line 239
Update OpenIDConnectClient.php5
Merge pull request #14 from cicalese/patch-4

Eliminate warning when $this->redirectURL has not been set