sys/event/callback: in init set list_node.next to NULL

When using static initializers uninitialized fields are set to 0, or
NULL for pointers. But when using event_callback_init() on non
static variables event_callback_t structure may hold non 0 values.
This will lead to the event never being called since if super.list_node.next
is not NULL as it is considered already in the event queue and therefore
not touched.

sys/event/callback.c

@@ -18,6 +18,7 @@ void _event_callback_handler(event_t *event)
 
 void event_callback_init(event_callback_t *event_callback, void (callback)(void *), void *arg)
 {
+    memset(event_callback, 0, sizeof(*event_callback));
     event_callback->super.handler = _event_callback_handler;
     event_callback->callback = callback;
     event_callback->arg = arg;