Version 1.17 Beta 1: Implemented a workaround for handling message headers...

Version 1.17 Beta 1: Implemented a workaround for handling message headers that are null (which are more common now with the message voting feature recently introduced in Synchronet).  Now such message headers won't cause weirdness in the message list.  Users won't be able to read such messages.  I'd like to find a way to not show such messages altogether in the message list - Will probably need to use the get_all_msg_headers() method in the MsgBase class to get all message headers except ones for vote messages.