Fixes: index -1 out of bounds for type 'AV1ReferenceFrameState [8]'
Fixes: 16079/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5758807440883712

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: James Almer <jamrial@gmail.com>
See: [FFmpeg-devel] [PATCH 05/13] avcodec/cbs_av1_syntax_template: Check ref_frame_idx before use
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>