Fix: src.ctf.lttng-live: return correct error status in lttng_live_msg_iter_init

Fix this clang-tidy warning:

    /home/simark/src/babeltrace/src/plugins/ctf/lttng-live/lttng-live.cpp:1802:9: warning: Value stored to 'status' is never read [clang-analyzer-deadcode.DeadStores]
            status = BT_MESSAGE_ITERATOR_CLASS_INITIALIZE_METHOD_STATUS_MEMORY_ERROR;
            ^        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Indeed, we set status to MEMORY_ERROR here, but overwrite it to ERROR at
the error label.  Remove the assignment at the error label, add
assignments to all points where we go to the error label.

Change-Id: I5a81210609d913854f7b7d1b1be5bd54fffa02b2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7195
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>