avfilter/zscale: fix memory leak

libavfilter/vf_zscale.c

@@ -673,6 +673,7 @@ static void uninit(AVFilterContext *ctx)
     ZScaleContext *s = ctx->priv;
 
     zimg_filter_graph_free(s->graph);
+    zimg_filter_graph_free(s->alpha_graph);
     av_freep(&s->tmp);
     s->tmp_size = 0;
 }