af_tempo: Add missing error check

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavfilter/af_atempo.c

@@ -1148,6 +1148,8 @@ static int request_frame(AVFilterLink *outlink)
 
             if (n_out) {
                 ret = push_samples(atempo, outlink, n_out);
+                if (ret < 0)
+                    return ret;
             }
         }