audio: fix a case of going to sleep before playback start

This code is tricky because it has to wakeup the mainloop to make
progressing during syncing audio, but also has to avoid waking it up
when it's not needed. Failure to do so either burns CPU by not ever
going to sleep, or causes apparent "freezes" by going to sleep (and it
will continue if the mainloop is woken up e.g. due to user input).

In this case, simply starting A/V playback with --start=5 and removing
an unrelated wakeup in osd.c can trigger such a "freeze". The unrelated
wakeup did hide this bug, nonetheless it's a bug.

(Can't wait to rewrite this shitty audio resync code. And it's all my
fault.)

player/audio.c

@@ -772,6 +772,7 @@ void fill_audio_out_buffers(struct MPContext *mpctx, double endpts)
         mp_audio_buffer_skip(ao_c->ao_buffer, MPMIN(skip, max));
         // If something is left, we definitely reached the target time.
         end_sync |= sync_known && skip < max;
+        working |= skip > 0;
     } else if (skip < 0) {
         if (-skip > playsize) { // heuristic against making the buffer too large
             ao_reset(mpctx->ao); // some AOs repeat data on underflow