avcodec/hevcdec: Check s->ref in the md5 path similar to hwaccel

This is somewhat redundant with the is_decoded check. Maybe
there is a nicer solution

Fixes: Null pointer dereference
Fixes: 49584/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5297367351427072

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

libavcodec/hevcdec.c

@@ -3498,7 +3498,7 @@ static int hevc_decode_frame(AVCodecContext *avctx, AVFrame *rframe,
         }
     } else {
         /* verify the SEI checksum */
-        if (avctx->err_recognition & AV_EF_CRCCHECK && s->is_decoded &&
+        if (avctx->err_recognition & AV_EF_CRCCHECK && s->ref && s->is_decoded &&
             s->sei.picture_hash.is_md5) {
             ret = verify_md5(s, s->ref->frame);
             if (ret < 0 && avctx->err_recognition & AV_EF_EXPLODE) {