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>
int check_tm_pred4x4_mode(int mode, int mb_x, int mb_y, int vp7)
{
    if (!mb_x) {
        return mb_y ? VERT_VP8_PRED : (vp7 ? DC_128_PRED : DC_129_PRED);
    } else {
        return mb_y ? mode : HOR_VP8_PRED;
    }
}

int check_tm_pred4x4_mode(int mode, int mb_x, int mb_y, int vp7)
{
    if (!mb_x) {
        return mb_y ? VERT_VP8_PRED : (vp7 ? DC_128_PRED : DC_129_PRED);
    } else {
        return mb_y ? mode : HOR_VP8_PRED;
    }
}
