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 av_cold int vp8_decode_init_thread_copy(AVCodecContext *avctx)
{
    VP8Context *s = avctx->priv_data;
    int ret;

    s->avctx = avctx;

    if ((ret = vp8_init_frames(s)) < 0) {
        ff_vp8_decode_free(avctx);
        return ret;
    }

    return 0;
}

static av_cold int vp8_decode_init_thread_copy(AVCodecContext *avctx)
{
    VP8Context *s = avctx->priv_data;
    int ret;

    s->avctx = avctx;

    if ((ret = vp8_init_frames(s)) < 0) {
        ff_vp8_decode_free(avctx);
        return ret;
    }

    return 0;
}
