Fix: flt-utils.muxer: reference leak in muxer_msg_iter_add_upstream_msg_iter error...

Fix: flt-utils.muxer: reference leak in muxer_msg_iter_add_upstream_msg_iter error path

Let's say we fail to allocate `muxer_upstream_msg_iter->msgs`, we will have
already gotten a reference on `self_msg_iter`, which we need to put.  Calling
destroy_muxer_upstream_msg_iter ensures we do that.

Change-Id: I9b113d2e335d529599cb9197c39c8675915508e5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3393
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>