Fixed infinite callback loop in check-session iframe

This commit fixes the JS callback defined in the check-session iframe which can produce infinite callback loops if the received message doesn't come from the relying
party. In that case another message is posted to the source of the message (which can be the OP itself) thus resulting in an infinite loop because "error" messages are
continuously generated by the callback function.

oidc_provider/templates/oidc_provider/check_session_iframe.html

@@ -9,6 +9,10 @@
             window.addEventListener("message", receiveMessage, false);
 
             function receiveMessage(e) {
+                if (!e.data || typeof e.data != 'string' || e.data == 'error') {
+                    return;
+                }
+
                 var status;
                 try {
                     var clientId = e.data.split(' ')[0];