avcodec/vvc/vvcdec: Do not submit frames without VVCFrameThread

Such frames will crash when pthread functions are called on the NULL pointer

Fixes: member access within null pointer of type 'VVCFrameThread' (aka 'struct VVCFrameThread')
Fixes: 65160/clusterfuzz-testcase-minimized-ffmpeg_BSF_VVC_METADATA_fuzzer-4665241535119360 (partly)
Fixes: 65636/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VVC_fuzzer-5394745824182272

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

libavcodec/vvc/vvcdec.c

@@ -940,6 +940,9 @@ static int vvc_decode_frame(AVCodecContext *avctx, AVFrame *output,
     if (ret < 0)
         return ret;
 
+    if (!fc->ft)
+        return avpkt->size;
+
     ret = submit_frame(s, fc, output, got_output);
     if (ret < 0)
         return ret;