Fix: src.ctf.lttng-live: session closed before any metadata is received

Fix: src.ctf.lttng-live: session closed before any metadata is received

Issue
=====
When stress-testing the LTTng-UST clear feature over an entire night I
witnessed 6 instances of Babeltrace infinite loop.

Using the GNU Debugger, Babeltrace was found to be endlessly issuing the
`LTTNG_VIEWER_GET_NEW_STREAMS` command to the Relay daemon to which the
Relay Daemon endlessly replied `LTTNG_VIEWER_NEW_STREAMS_HUP`.

This is caused by the fact that the session was closed before we
received any metadata and the lttng-live component class keeps trying to
ask for new streams before requesting the metadata.

Fix
===
If the session is marked as closed return a `_STATUS_END` to stop
considering this session.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9c18d03cef1b61d1c02b7d608720bf5750a348c4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7390