vf_aspect: clear AVFilterLink.cur_buf in start_frame().

The buffer is passed on to the next filter, so we shouldn't keep any
pointers to it.

libavfilter/vf_aspect.c

@@ -69,6 +69,7 @@ static void start_frame(AVFilterLink *link, AVFilterBufferRef *picref)
     AspectContext *aspect = link->dst->priv;
 
     picref->video->pixel_aspect = aspect->aspect;
+    link->cur_buf = NULL;
     ff_start_frame(link->dst->outputs[0], picref);
 }