avfilter/avf_showspectrum: use proper function return value instead of hardcoded number

libavfilter/avf_showspectrum.c

@@ -1037,7 +1037,7 @@ static int config_output(AVFilterLink *outlink)
     }
 
     s->win_size = fft_size;
-    s->buf_size = FFALIGN(s->win_size << (!!s->stop), 512);
+    s->buf_size = FFALIGN(s->win_size << (!!s->stop), av_cpu_max_align());
 
     if (!s->fft) {
         s->fft = av_calloc(inlink->channels, sizeof(*s->fft));