BUG/MEDIUM: logs: fix http-client's log srv initialization

BUG/MEDIUM: logs: fix http-client's log srv initialization

As anticipated in commit 211ea252d ("BUG/MINOR: logs: fix logsrv leaks
on clean exit"), there were indeed other corner cases that were not
properly covered. Setting the http client's ring_name to NULL make the
sink lookup crash on startup in sink_find () with a config as simple as:

    global
        log ring@buf0 local0

The fields must be properly initialized (both config file name and
the ring_name). This only needs to be backported if/when the commit
above is backported.