Fix: live_viewer_connection_create(): `lttng_live_msg_iter` can be `NULL`

Fix: live_viewer_connection_create(): `lttng_live_msg_iter` can be `NULL`

Because `lttng_live_msg_iter` can be `NULL` in
live_viewer_connection_create() when it's used from a query operation,
pass the log level to use directly and only read `self_comp` if
`lttng_live_msg_iter` exists.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ied5ec621bcd066df0bb312a6f81899616f9d1a9f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1797