Fix NPE in IQDiscoItemsHandler when an ServerItemsProvider is disabled

According to ServerItemsProvider javadoc it is possible for #getItems to return null. So we need to consider that case in IQDiscoItemsHandler .

2006.05.07 18:11:50 [org.jivesoftware.wildfire.XMPPServer.startModules(XMPPServer.java:504)
] Internal server error
java.lang.NullPointerException
at org.jivesoftware.wildfire.disco.IQDiscoItemsHandler.addServerItemsProvider(IQDiscoIt emsHandler.java:178)
at org.jivesoftware.wildfire.disco.IQDiscoItemsHandler.start(IQDiscoItemsHandler.java:2 70)
at org.jivesoftware.wildfire.XMPPServer.startModules(XMPPServer.java:497)
at org.jivesoftware.wildfire.XMPPServer.start(XMPPServer.java:371)
at org.jivesoftware.wildfire.XMPPServer.<init>(XMPPServer.java:142)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at java.lang.Class.newInstance0(Unknown Source)
at java.lang.Class.newInstance(Unknown Source)
at org.jivesoftware.wildfire.starter.ServerStarter.start(ServerStarter.java:88)
at org.jivesoftware.wildfire.starter.ServerStarter.main(ServerStarter.java:49)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.exe4j.runtime.LauncherEngine.launch(Unknown Source)
at com.exe4j.runtime.WinLauncher.main(Unknown Source)