avcodec/vvc/vvcdec: allow vvc_decode_close() on failed open

Fixes: division by 0
Fixes: 67008/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VVC_fuzzer-5873503163187200

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vvc/vvcdec.c

@@ -950,13 +950,14 @@ static av_cold void vvc_decode_flush(AVCodecContext *avctx)
 {
     VVCContext *s  = avctx->priv_data;
     int got_output = 0;
-    VVCFrameContext *last;
 
     while (s->nb_delayed)
         wait_delayed_frame(s, NULL, &got_output);
 
-    last = get_frame_context(s, s->fcs, s->nb_frames - 1);
-    ff_vvc_flush_dpb(last);
+    if (s->fcs) {
+        VVCFrameContext *last = get_frame_context(s, s->fcs, s->nb_frames - 1);
+        ff_vvc_flush_dpb(last);
+    }
 
     s->eos = 1;
 }