setup.sh: Fix ArrayIndexOutOfBoundsException / NPE

`WEBINF/setup.sh` without parameters ends abruptly without printing any help message. See output:

````
23427:	Admin@Offline>exit
23428:	
23429:	
23430:	The setup is finished!
23431:	The OpenCms system used for the setup will now shut down.
23432:	
23433:	
23434:	Shutting down OpenCms, version 9.5.x in web application "opencms"
23435:	Shutdown completed, total uptime was 00:05:17.
23436:	

-------------------------------------------------
Setup started at: (Mon Oct 06 10:01:58 CEST 2014)
-------------------------------------------------

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
	at org.opencms.setup.CmsAutoSetup.main(CmsAutoSetup.java:92)
````

`WEB-INF/setup.sh -help` or `WEB-INF/setup.sh --help` also breaks with a NPE:

````
23427:	Admin@Offline>exit
23428:	
23429:	
23430:	The setup is finished!
23431:	The OpenCms system used for the setup will now shut down.
23432:	
23433:	
23434:	Shutting down OpenCms, version 9.5.x in web application "opencms"
23435:	Shutdown completed, total uptime was 00:05:17.
23436:	

-------------------------------------------------
Setup started at: (Mon Oct 06 10:04:18 CEST 2014)
-------------------------------------------------

Exception in thread "main" java.lang.NullPointerException
	at java.io.File.<init>(File.java:277)
	at org.opencms.setup.CmsAutoSetup.main(CmsAutoSetup.java:100)
````