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 void chroma_mc_uni(HEVCContext *s, uint8_t *dst0,
                          ptrdiff_t dststride, uint8_t *src0, ptrdiff_t srcstride, int reflist,
                          int x_off, int y_off, int block_w, int block_h, struct MvField *current_mv, int chroma_weight, int chroma_offset)
{
    HEVCLocalContext *lc = s->HEVClc;
    int pic_width        = s->ps.sps->width >> s->ps.sps->hshift[1];
    int pic_height       = s->ps.sps->height >> s->ps.sps->vshift[1];
    const Mv *mv         = &current_mv->mv[reflist];
    int weight_flag      = (s->sh.slice_type == HEVC_SLICE_P && s->ps.pps->weighted_pred_flag) ||
                           (s->sh.slice_type == HEVC_SLICE_B && s->ps.pps->weighted_bipred_flag);
    int idx              = ff_hevc_pel_weight[block_w];
    int hshift           = s->ps.sps->hshift[1];
    int vshift           = s->ps.sps->vshift[1];
    intptr_t mx          = av_mod_uintp2(mv->x, 2 + hshift);
    intptr_t my          = av_mod_uintp2(mv->y, 2 + vshift);
    intptr_t _mx         = mx << (1 - hshift);
    intptr_t _my         = my << (1 - vshift);

    x_off += mv->x >> (2 + hshift);
    y_off += mv->y >> (2 + vshift);
    src0  += y_off * srcstride + (x_off * (1 << s->ps.sps->pixel_shift));

    if (x_off < EPEL_EXTRA_BEFORE || y_off < EPEL_EXTRA_AFTER ||
        x_off >= pic_width - block_w - EPEL_EXTRA_AFTER ||
        y_off >= pic_height - block_h - EPEL_EXTRA_AFTER) {
        const int edge_emu_stride = EDGE_EMU_BUFFER_STRIDE << s->ps.sps->pixel_shift;
        int offset0 = EPEL_EXTRA_BEFORE * (srcstride + (1 << s->ps.sps->pixel_shift));
        int buf_offset0 = EPEL_EXTRA_BEFORE *
                          (edge_emu_stride + (1 << s->ps.sps->pixel_shift));
        s->vdsp.emulated_edge_mc(lc->edge_emu_buffer, src0 - offset0,
                                 edge_emu_stride, srcstride,
                                 block_w + EPEL_EXTRA, block_h + EPEL_EXTRA,
                                 x_off - EPEL_EXTRA_BEFORE,
                                 y_off - EPEL_EXTRA_BEFORE,
                                 pic_width, pic_height);

        src0 = lc->edge_emu_buffer + buf_offset0;
        srcstride = edge_emu_stride;
    }
    if (!weight_flag)
        s->hevcdsp.put_hevc_epel_uni[idx][!!my][!!mx](dst0, dststride, src0, srcstride,
                                                  block_h, _mx, _my, block_w);
    else
        s->hevcdsp.put_hevc_epel_uni_w[idx][!!my][!!mx](dst0, dststride, src0, srcstride,
                                                        block_h, s->sh.chroma_log2_weight_denom,
                                                        chroma_weight, chroma_offset, _mx, _my, block_w);
}

static void chroma_mc_uni(HEVCContext *s, uint8_t *dst0,
                          ptrdiff_t dststride, uint8_t *src0, ptrdiff_t srcstride, int reflist,
                          int x_off, int y_off, int block_w, int block_h, struct MvField *current_mv, int chroma_weight, int chroma_offset)
{
    HEVCLocalContext *lc = s->HEVClc;
    int pic_width        = s->ps.sps->width >> s->ps.sps->hshift[1];
    int pic_height       = s->ps.sps->height >> s->ps.sps->vshift[1];
    const Mv *mv         = &current_mv->mv[reflist];
    int weight_flag      = (s->sh.slice_type == HEVC_SLICE_P && s->ps.pps->weighted_pred_flag) ||
                           (s->sh.slice_type == HEVC_SLICE_B && s->ps.pps->weighted_bipred_flag);
    int idx              = ff_hevc_pel_weight[block_w];
    int hshift           = s->ps.sps->hshift[1];
    int vshift           = s->ps.sps->vshift[1];
    intptr_t mx          = av_mod_uintp2(mv->x, 2 + hshift);
    intptr_t my          = av_mod_uintp2(mv->y, 2 + vshift);
    intptr_t _mx         = mx << (1 - hshift);
    intptr_t _my         = my << (1 - vshift);

    x_off += mv->x >> (2 + hshift);
    y_off += mv->y >> (2 + vshift);
    src0  += y_off * srcstride + (x_off * (1 << s->ps.sps->pixel_shift));

    if (x_off < EPEL_EXTRA_BEFORE || y_off < EPEL_EXTRA_AFTER ||
        x_off >= pic_width - block_w - EPEL_EXTRA_AFTER ||
        y_off >= pic_height - block_h - EPEL_EXTRA_AFTER) {
        const int edge_emu_stride = EDGE_EMU_BUFFER_STRIDE << s->ps.sps->pixel_shift;
        int offset0 = EPEL_EXTRA_BEFORE * (srcstride + (1 << s->ps.sps->pixel_shift));
        int buf_offset0 = EPEL_EXTRA_BEFORE *
                          (edge_emu_stride + (1 << s->ps.sps->pixel_shift));
        s->vdsp.emulated_edge_mc(lc->edge_emu_buffer, src0 - offset0,
                                 edge_emu_stride, srcstride,
                                 block_w + EPEL_EXTRA, block_h + EPEL_EXTRA,
                                 x_off - EPEL_EXTRA_BEFORE,
                                 y_off - EPEL_EXTRA_BEFORE,
                                 pic_width, pic_height);

        src0 = lc->edge_emu_buffer + buf_offset0;
        srcstride = edge_emu_stride;
    }
    if (!weight_flag)
        s->hevcdsp.put_hevc_epel_uni[idx][!!my][!!mx](dst0, dststride, src0, srcstride,
                                                  block_h, _mx, _my, block_w);
    else
        s->hevcdsp.put_hevc_epel_uni_w[idx][!!my][!!mx](dst0, dststride, src0, srcstride,
                                                        block_h, s->sh.chroma_log2_weight_denom,
                                                        chroma_weight, chroma_offset, _mx, _my, block_w);
}
