BUG/MINOR: hlua: assume proxy_find_by_id() may return NULL on previously existing...

BUG/MINOR: hlua: assume proxy_find_by_id() may return NULL on previously existing server

Since 3.4 and dynamic backends, we may hit a race were a backend which
existed at the time the event was published does not exist anymore when
the event is processed. Previously it was assumed the proxy referenced
by a server event would always exist (since proxies couldn't be removed
dynamically), and a BUG_ON() ensured that. But it is not true anymore,
thus we need to check for proxy_find_by_id() NULL return value, which
indicates that the proxy was removed since, and thus it is not worth
looking for the server which triggered the event anyway.

It should be backported to 3.4.