Fix: ctf-writer: null dereference in bt_ctf_trace_common_add_stream_class

gcc 12.1.0 reports that:
In file included from object-pool.h:38,
                 from clock-class.h:11,
                 from trace.c:27:
In function 'bt_ctf_object_set_parent',
    inlined from 'bt_ctf_object_set_parent' at object.h:102:6,
    inlined from 'bt_ctf_trace_common_add_stream_class' at trace.c:1227:3:
object.h:123:26: warning: null pointer dereference [-Wnull-dereference]
  123 |                 if (child->parent) {
      |                     ~~~~~^~~~~~~~
object.h:123:26: warning: null pointer dereference [-Wnull-dereference]

This can indeed happen if bt_ctf_trace_common_add_stream_class is called
with a nil stream class argument. A null check is added in the error
path.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I313966c1747c3929a46b98af71ba58f607d3c7df
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8084