Fix: do not set event's parent to a non-writer stream

Fix: do not set event's parent to a non-writer stream

Since a non-writer stream does not know the events that are
associated to it, an event with a non-writer stream parent can
never be destroyed.

Fix this by setting a stream parent if the stream is a writer
stream, and otherwise by using the event's packet's stream.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>