Cleanup: flt.utils.muxer: avoid returning a pointer needlessly

Reshuffle `muxer_msg_iter_add_upstream_msg_iter()` function to return an
int rather than a pointer.

Returning a pointer in this case has no value since the newly allocated
structure is stored in a GQueue directly. Returning an int to express
success or error seems more appropriate and less error-prone.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I111cd06e1f667c98aa3bf650375c6e49fcd4c5dd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1313
Tested-by: jenkins
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>