avfilter/af_amix: dont fail if there are no samples in output_frame()

Fixes Ticket5326
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_amix.c

@@ -312,6 +312,9 @@ static int output_frame(AVFilterLink *outlink)
 
     calculate_scales(s, nb_samples);
 
+    if (nb_samples == 0)
+        return 0;
+
     out_buf = ff_get_audio_buffer(outlink, nb_samples);
     if (!out_buf)
         return AVERROR(ENOMEM);