Fix: src.ctf.lttng-live: emitting stream end msg with no stream

Fix: src.ctf.lttng-live: emitting stream end msg with no stream

Background
==========
When a stream hangs up on the `src.ctf.lttng-live` component, we make
sure we send a stream end message to ensure we honor The Contract which
states that any stream beginning must eventually be followed by its
stream end counterpart. We do this by calling
`ctf_msg_iter_get_next_message()` one last time to emit any missing
messages.

Using the upcoming lttng clear feature in conjunction with a per-pid
session makes it highly likely that a live stream hangs up on the
`src.ctf.lttng-live` component between the moment we learn about it and
the moment we first ask for its live index.

In such event, the live stream iterator and its `ctf_msg_it` are both
created but the corresponding stream is uninitialized.

When the component realized that a live stream has hung up, it calls
`ctf_msg_iter_get_next_message()` to respect The Contract but then
errors out here:
  CAUSED BY [lttng-live: 'source.ctf.lttng-live'] (msg-iter.c:2474)
    Cannot create stream end message because stream is NULL:
    msg-it-addr=0x555fba864010

The `stream` field is null because we never got the chance to received
any index for this stream.

Issue
=====
It's possible for a `ctf_msg` state machine to pass by the
`STATE_EMIT_MSG_STREAM_END` state without having passed by the
`STATE_EMIT_MSG_STREAM_BEGINNING` state.

Solution
========
Keep track of the fact that we sent a stream beginning message
downstream and that we need to send its respective stream end message.
If no message were send for a particular stream, we can omit sending a
stream end message.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If7f52f43162e7263785713c01c226907fe475d94
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2719
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>