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: 280c1e1cSigned-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180724102215.31866-1-dgilbert@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

hw/audio/hda-codec.c

@@ -786,7 +786,7 @@ static void hda_audio_reset(DeviceState *dev)
 static bool vmstate_hda_audio_stream_buf_needed(void *opaque)
 {
     HDAAudioStream *st = opaque;
-    return st->state->use_timer;
+    return st->state && st->state->use_timer;
 }
 
 static const VMStateDescription vmstate_hda_audio_stream_buf = {