Fix: flt.utils.muxer: Potential memory leak

Fix: flt.utils.muxer: Potential memory leak

scan-build reports a potential memory leak of the
muxer_upstream_msg_iter pointer allocated in the
`muxer_msg_iter_add_upstream_msg_iter()` function if the `g_queue_new()`
fails.

So, free the muxer_upstream_msg_iter on the error path.

Reported-by: scan-build - Potential leak of memory
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ia37a1be9525b357d063d5c5bd5623ddccbd4e29d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1312
Tested-by: jenkins
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>