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_4(uint8_t *p, const uint8_t *p_l, const uint8_t *p_tl,
                          const uint8_t *p_t, const uint8_t *p_tr)
{
    AV_COPY32(p, p_tl);
}

static void inv_predict_4(uint8_t *p, const uint8_t *p_l, const uint8_t *p_tl,
                          const uint8_t *p_t, const uint8_t *p_tr)
{
    AV_COPY32(p, p_tl);
}
