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 set_deblocking_bypass(HEVCContext *s, int x0, int y0, int log2_cb_size)
{
    int cb_size          = 1 << log2_cb_size;
    int log2_min_pu_size = s->ps.sps->log2_min_pu_size;

    int min_pu_width     = s->ps.sps->min_pu_width;
    int x_end = FFMIN(x0 + cb_size, s->ps.sps->width);
    int y_end = FFMIN(y0 + cb_size, s->ps.sps->height);
    int i, j;

    for (j = (y0 >> log2_min_pu_size); j < (y_end >> log2_min_pu_size); j++)
        for (i = (x0 >> log2_min_pu_size); i < (x_end >> log2_min_pu_size); i++)
            s->is_pcm[i + j * min_pu_width] = 2;
}

static void set_deblocking_bypass(HEVCContext *s, int x0, int y0, int log2_cb_size)
{
    int cb_size          = 1 << log2_cb_size;
    int log2_min_pu_size = s->ps.sps->log2_min_pu_size;

    int min_pu_width     = s->ps.sps->min_pu_width;
    int x_end = FFMIN(x0 + cb_size, s->ps.sps->width);
    int y_end = FFMIN(y0 + cb_size, s->ps.sps->height);
    int i, j;

    for (j = (y0 >> log2_min_pu_size); j < (y_end >> log2_min_pu_size); j++)
        for (i = (x0 >> log2_min_pu_size); i < (x_end >> log2_min_pu_size); i++)
            s->is_pcm[i + j * min_pu_width] = 2;
}
