opusenc: fix infinite loop if flushing encoder upon init

The issue is that the afq still has samples as on init it counts
the overlap used as a delay to adjust the PTS it generates, hence
we can't rely on it right after init.
So just check to see if any frames have been encoded. frame_number
can't be anything but 0 right after init and can only be set by lavc.
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/opusenc.c

@@ -543,7 +543,7 @@ static int opus_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
         ff_bufqueue_add(avctx, &s->bufqueue, av_frame_clone(frame));
     } else {
         ff_opus_psy_signal_eof(&s->psyctx);
-        if (!s->afq.remaining_samples)
+        if (!s->afq.remaining_samples || !avctx->frame_number)
             return 0; /* We've been flushed and there's nothing left to encode */
     }