avcodec/h264dec: Clear ref_count on slice header processing failure

Fixes using freed memory
Introduced in 744801989099df26e90b00062c645969c5347533
Fixes: 471/fuzz-1-ffmpeg_VIDEO_AV_CODEC_ID_H264_fuzzer

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