audio/hda: Fix migration

Fix outgoing migration which was crashing in
vmstate_hda_audio_stream_buf_needed, I think the problem
is that we have room for upto 4 streams in the array but only
use 2, when we come to try and save the state of the unused
streams we hit st->state == NULL.

Fixes: 280c1e1c
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180724102215.31866-1-dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>