avfilter/buffer: use av_freep() to avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/buffer.c

@@ -38,7 +38,7 @@ void ff_avfilter_default_free_buffer(AVFilterBuffer *ptr)
 {
     if (ptr->extended_data != ptr->data)
         av_freep(&ptr->extended_data);
-    av_free(ptr->data[0]);
+    av_freep(&ptr->data[0]);
     av_free(ptr);
 }