defensive: NULL-check calloc results before dereferencing

Five sites allocated with calloc() and dereferenced the result on the
very next line. Under OOM the module would have segfaulted instead of
degrading. Each site now bails (or sends a Canceled D-Bus error, in
the agent path) when the allocation fails.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/iwd/iwd_manager.c

@@ -63,6 +63,7 @@ iwd_manager_listener_add(Iwd_Manager *m, Iwd_Manager_Cb cb, void *data)
 {
    if (!m || !cb) return;
    Listener *l = calloc(1, sizeof(*l));
+   if (!l) return;
    l->cb = cb; l->data = data;
    m->listeners = eina_list_append(m->listeners, l);
 }