avcodec/h264_cavlc: Fix runtime error: index -1 out of bounds for type 'VLC [15]

Fixes: 1513/clusterfuzz-testcase-minimized-6246484833992704

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