avfilter/vf_format: add .get_video_buffer back

Unbreak xyz12 with vflip.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_format.c

@@ -116,6 +116,7 @@ static const AVFilterPad avfilter_vf_format_inputs[] = {
     {
         .name             = "default",
         .type             = AVMEDIA_TYPE_VIDEO,
+        .get_video_buffer = ff_null_get_video_buffer,
     },
     { NULL }
 };
@@ -158,6 +159,7 @@ static const AVFilterPad avfilter_vf_noformat_inputs[] = {
     {
         .name             = "default",
         .type             = AVMEDIA_TYPE_VIDEO,
+        .get_video_buffer = ff_null_get_video_buffer,
     },
     { NULL }
 };