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 inv_predict_11(uint8_t *p, const uint8_t *p_l, const uint8_t *p_tl,
                           const uint8_t *p_t, const uint8_t *p_tr)
{
    int diff = (FFABS(p_l[0] - p_tl[0]) - FFABS(p_t[0] - p_tl[0])) +
               (FFABS(p_l[1] - p_tl[1]) - FFABS(p_t[1] - p_tl[1])) +
               (FFABS(p_l[2] - p_tl[2]) - FFABS(p_t[2] - p_tl[2])) +
               (FFABS(p_l[3] - p_tl[3]) - FFABS(p_t[3] - p_tl[3]));
    if (diff <= 0)
        AV_COPY32(p, p_t);
    else
        AV_COPY32(p, p_l);
}

static void inv_predict_11(uint8_t *p, const uint8_t *p_l, const uint8_t *p_tl,
                           const uint8_t *p_t, const uint8_t *p_tr)
{
    int diff = (FFABS(p_l[0] - p_tl[0]) - FFABS(p_t[0] - p_tl[0])) +
               (FFABS(p_l[1] - p_tl[1]) - FFABS(p_t[1] - p_tl[1])) +
               (FFABS(p_l[2] - p_tl[2]) - FFABS(p_t[2] - p_tl[2])) +
               (FFABS(p_l[3] - p_tl[3]) - FFABS(p_t[3] - p_tl[3]));
    if (diff <= 0)
        AV_COPY32(p, p_t);
    else
        AV_COPY32(p, p_l);
}
