avcodec/vvcdec: frame_context_setup, set fc->ref to NULL

fc->ref points to an old VVCFrame, which cannot be used after frame_context_setup.
This prevents crashes in decode_nal_units-->ff_vvc_report_frame_finished.

Signed-off-by: Frank Plowman <post@frankplowman.com>

libavcodec/vvc/vvcdec.c

@@ -594,6 +594,8 @@ static int frame_context_setup(VVCFrameContext *fc, VVCContext *s)
 {
     int ret;
 
+    fc->ref = NULL;
+
     // copy refs from the last frame
     if (s->nb_frames && s->nb_fcs > 1) {
         VVCFrameContext *prev = get_frame_context(s, fc, -1);