purple: Fix handling of empty, immediate roomlist results
Two issues here:
- SIPE called in_progress(FALSE) immediately (which decreases refcount),
before purple_roomlist_get_list() could return (which would normally
increase refcount). The first refcount decrease steals it from the prpl,
and bad things happen.
Added an initialized flag to only do that decrease after it was
increased first. This is similar to how pidgin sets a 'dialog' attribute
after the purple_roomlist_get_list() call, and skips the unref if it's
not set.
- The code assumed that NULL return value means room listing not
supported. That's not quite true, so now it checks in the prpl info to
see if roomlist_get_list is defined.
Also, made purple_roomlist_data more private.
