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 void vp8_decode_flush_impl(AVCodecContext *avctx, int free_mem)
{
    VP8Context *s = avctx->priv_data;
    int i;

    for (i = 0; i < FF_ARRAY_ELEMS(s->frames); i++)
        vp8_release_frame(s, &s->frames[i]);
    memset(s->framep, 0, sizeof(s->framep));

    if (free_mem)
        free_buffers(s);
}

static void vp8_decode_flush_impl(AVCodecContext *avctx, int free_mem)
{
    VP8Context *s = avctx->priv_data;
    int i;

    for (i = 0; i < FF_ARRAY_ELEMS(s->frames); i++)
        vp8_release_frame(s, &s->frames[i]);
    memset(s->framep, 0, sizeof(s->framep));

    if (free_mem)
        free_buffers(s);
}
