avfilter: correct memcpy size avfilter_copy_buf_props()

Does not make a difference in this case since sizeof(uint8_t*) ==
sizeof(uint8_t**). Fixes CID703814.

libavfilter/buffer.c

@@ -146,7 +146,7 @@ int avfilter_copy_buf_props(AVFrame *dst, const AVFilterBufferRef *src)
             if (!dst->extended_data)
                 return AVERROR(ENOMEM);
             memcpy(dst->extended_data, src->extended_data,
-                   planes * sizeof(dst->extended_data));
+                   planes * sizeof(*dst->extended_data));
         } else
             dst->extended_data = dst->data;