ffplay: do not erase the content of vfilters

This way the content of "vfilters" can be reused.

For example when the frame size changes, the filterchain is
reconfigured reusing again the vfilters value.

ffplay.c

@@ -1702,7 +1702,6 @@ static int configure_video_filters(AVFilterGraph *graph, VideoState *is, const c
 
         if ((ret = avfilter_graph_parse(graph, vfilters, &inputs, &outputs, NULL)) < 0)
             return ret;
-        av_freep(&vfilters);
     } else {
         if ((ret = avfilter_link(filt_src, 0, filt_out, 0)) < 0)
             return ret;