Fix: lib/graph: Dereference before null checks

Fix: lib/graph: Dereference before null checks

  CID 1401244 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.source_output_port_added suggests that it may be null,
  but it has already been dereferenced on all paths leading to the check.

  CID 1401260 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.filter_output_port_added suggests that it may be null,
  but it has already been dereferenced on all paths leading to the check.k

  CID 1401243 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.filter_input_port_added suggests that it may be null,
  but it has already been dereferenced on all paths leading to the check.

  CID 1401185 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking graph->listeners.sink_input_port_added
  suggests that it may be null, but it has already been dereferenced on
  all paths leading to the check.

  CID 1401262 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.source_filter_ports_connected suggests that it may be
  null, but it has already been dereferenced on all paths leading to the
  check.

  CID 1401200 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.filter_filter_ports_connected suggests that it may be
  null, but it has already been dereferenced on all paths leading to the
  check.

  CID 1401252 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.source_sink_ports_connected suggests that it may be
  null, but it has already been dereferenced on all paths leading to the
  check.

  CID 1401264 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking
  graph->listeners.filter_sink_ports_connected suggests that it may be
  null, but it has already been dereferenced on all paths leading to the
  check.

Reported-by: Coverity (various ids) Dereference before null check
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I88c8172e006e4bae2c7b5e29b501e1a36f19ae5c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1262
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>