chatty-conversation: Don't crash on conversation with not yet buddies

If someone sends messages but is not yet a buddy chatty would crash since purple_find_buddy returns NULL. Handle that by using the conversation name.

The individual commits have more details. I'm also happy to call purple_blist_add_buddy() if that makes more sense but this is usually triggered by XMPP spam which I rather not have end up in my buddy list.

Closes: #82 (closed)