PulseAudio: fix audio drop after a flush

It happens when pa_stream_write is called too late after a flush.
This fixes audio drop when the track has a positive delay.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit 7de1c701a5a43fd5a5cbd236fa64d750a356c7de)

modules/audio_output/pulse.c

@@ -561,6 +561,9 @@ static void Flush(audio_output_t *aout, bool wait)
         op = pa_stream_flush(s, NULL, NULL);
     if (op != NULL)
         pa_operation_unref(op);
+    sys->first_pts = VLC_TS_INVALID;
+    stream_stop(s, aout);
+
     pa_threaded_mainloop_unlock(sys->mainloop);
 }