Nullpointer on nonexistent session (#58)

When Jenkins is restarted(or session information otherwise lost) half way through the interaction it might not have a session at this point.

src/main/java/org/jenkinsci/plugins/oic/OicSecurityRealm.java

@@ -619,7 +619,12 @@ private String buildOAuthRedirectUrl() throws NullPointerException {
      * @return an HttpResponse
     */
     public HttpResponse doFinishLogin(StaplerRequest request) {
-        return OicSession.getCurrent().doFinishLogin(request);
+    	OicSession currentSession = OicSession.getCurrent();
+    	if(currentSession==null) {
+    		LOGGER.fine("No session to resume (perhaps jenkins was restarted?)");
+    		return HttpResponses.errorWithoutStack(401, "Unauthorized");
+    	}
+        return currentSession.doFinishLogin(request);
     }
 
     /**