Adds argument checks for null value in getExistingChatRooms in MUC

Service. Patch provided by Danny van Heumen.

src/net/java/sip/communicator/impl/muc/MUCServiceImpl.java

@@ -478,6 +478,9 @@ public ChatRoomWrapper createPrivateChatRoom(
     public List<String> getExistingChatRooms(
         ChatRoomProviderWrapper chatRoomProvider)
     {
+        if (chatRoomProvider == null)
+            return null;
+        
         ProtocolProviderService protocolProvider
             = chatRoomProvider.getProtocolProvider();