af_amerge: compute output timestamp.

Use the timestamps from the first input.

libavfilter/af_amerge.c

@@ -216,6 +216,11 @@ static void filter_samples(AVFilterLink *inlink, AVFilterBufferRef *insamples)
         ins[i] = (*inbuf[i])->data[0] +
                  am->queue[i].pos * am->nb_in_ch[i] * am->bps;
     }
+    outbuf->pts = (*inbuf[0])->pts == AV_NOPTS_VALUE ? AV_NOPTS_VALUE :
+                  (*inbuf[0])->pts +
+                  av_rescale_q(am->queue[0].pos,
+                               (AVRational){ 1, ctx->inputs[0]->sample_rate },
+                               ctx->outputs[0]->time_base);
 
     avfilter_copy_buffer_ref_props(outbuf, *inbuf[0]);
     outbuf->audio->nb_samples     = nb_samples;