avcodec/hevcdec: Avoid only partly skiping duplicate first slices

Fixes: NULL pointer dereference and out of array access
Fixes: 13871/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5746167087890432
Fixes: 13845/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5650370728034304

This also fixes the return code for explode mode

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static int hls_slice_data(HEVCContext *s)
{
    int arg[2];
    int ret[2];

    arg[0] = 0;
    arg[1] = 1;

    s->avctx->execute(s->avctx, hls_decode_entry, arg, ret , 1, sizeof(int));
    return ret[0];
}

static int hls_slice_data(HEVCContext *s)
{
    int arg[2];
    int ret[2];

    arg[0] = 0;
    arg[1] = 1;

    s->avctx->execute(s->avctx, hls_decode_entry, arg, ret , 1, sizeof(int));
    return ret[0];
}
