avcodec/webp: Always set pix_fmt

Fixes: out of array access
Fixes: 1434/clusterfuzz-testcase-minimized-6314998085189632
Fixes: 1435/clusterfuzz-testcase-minimized-6483783723253760

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static int parse_transform_predictor(WebPContext *s)
{
    int block_bits, blocks_w, blocks_h, ret;

    PARSE_BLOCK_SIZE(s->width, s->height);

    ret = decode_entropy_coded_image(s, IMAGE_ROLE_PREDICTOR, blocks_w,
                                     blocks_h);
    if (ret < 0)
        return ret;

    s->image[IMAGE_ROLE_PREDICTOR].size_reduction = block_bits;

    return 0;
}

static int parse_transform_predictor(WebPContext *s)
{
    int block_bits, blocks_w, blocks_h, ret;

    PARSE_BLOCK_SIZE(s->width, s->height);

    ret = decode_entropy_coded_image(s, IMAGE_ROLE_PREDICTOR, blocks_w,
                                     blocks_h);
    if (ret < 0)
        return ret;

    s->image[IMAGE_ROLE_PREDICTOR].size_reduction = block_bits;

    return 0;
}
