audio: fix A/V sync in encoding mode

In encoding mode, the AO pretends to be infinitely fast (it will take
whatever we write, without ever rejecting input). Commit 261506e3 broke
this somehow. It turns out an old hack dealing with this was accidentally
dropped.

This is the hunk of code whose semantics were (partially) dropped:

    if (mpctx->d_audio && (mpctx->restart_playback ? !video_left :
                           ao_untimed(mpctx->ao) && (mpctx->delay <= 0 ||
                                                     !video_left)))
    {
        int status = fill_audio_out_buffers(mpctx, endpts);
        // Not at audio stream EOF yet
        audio_left = status > -2;
    }

This if condition is pretty wild, and it looked like it was pretty much
for audio-only mode, rather than subtle handling for encoding mode.