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_dc_pred8x8_mode(int mode, int mb_x, int mb_y)
{
    if (!mb_x)
        return mb_y ? TOP_DC_PRED8x8 : DC_128_PRED8x8;
    else
        return mb_y ? mode : LEFT_DC_PRED8x8;
}

int check_dc_pred8x8_mode(int mode, int mb_x, int mb_y)
{
    if (!mb_x)
        return mb_y ? TOP_DC_PRED8x8 : DC_128_PRED8x8;
    else
        return mb_y ? mode : LEFT_DC_PRED8x8;
}
