opus: Buffer the samples from the correct offset

When not all the opus stream have the same amount of decoded samples
process the least amount and store what is left from the other streams.

Bug-Id: 909
CC: libav-stable@libav.org
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/opusdec.c

@@ -567,8 +567,8 @@ static int opus_decode_packet(AVCodecContext *avctx, void *data,
         if (buffer_samples) {
             float *buf[2] = { c->out[2 * i + 0] ? c->out[2 * i + 0] : (float*)frame->extended_data[0],
                               c->out[2 * i + 1] ? c->out[2 * i + 1] : (float*)frame->extended_data[0] };
-            buf[0] += buffer_samples;
-            buf[1] += buffer_samples;
+            buf[0] += decoded_samples;
+            buf[1] += decoded_samples;
             ret = av_audio_fifo_write(c->sync_buffers[i], (void**)buf, buffer_samples);
             if (ret < 0)
                 return ret;