Fix: ctf: notif-iter: error on empty events (0 bits of data)

Fix: ctf: notif-iter: error on empty events (0 bits of data)

Do not create an event which consumed 0 bits of data stream: log an
error and return without creating it. We keep the buffer position at the
beginning of the last decoded event header, and we compare to the
current buffer position: if they are the same, then we consumed 0 bits
of data. It is possible, when we compare, that notit->buf.last_eh_at is
-1ULL (reset when requesting bytes from medium): in this case, we know
we requested bytes anyway, which means we consumed something to get to
this starvation point, thus the event is not empty.

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