[FIX] mail: traceback on new msg in unpinned closed livechat

The traceback ocurred when the operator closed a livechat channel chat window,
and then unpinned it from the sidebar in Discuss. Then, if the visitor sent
him a message, the channel was automatically re-pinned to the operator, who
received two notifications: the first one being the new channel info and the
second one the message itself.

The channel fold_state being 'closed', the window manager was trying to close
it again, which produces the traceback as the channel didn't exist in the JS
cache anymore (it was re-added to the cache when processing the message
notification, so just after).

Anyway, closing this channel was useless as, even if it was in the cache, it
would already been closed. So this rev. simply checks whether or not the
channel is in the cache before trying to close it.