Fix: don't use a return code in _bt_ctf_stream_append_discarded_events

That function is defined as void:
| File "/usr/lib/python3/dist-packages/babeltrace.py", line 2284, in
| append_discarded_events
| if ret < 0:
| TypeError: unorderable types: NoneType() < int()

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>