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>
av_cold int ff_vp8_decode_init(AVCodecContext *avctx)
{
    return vp78_decode_init(avctx, IS_VP8);
}

av_cold int ff_vp8_decode_init(AVCodecContext *avctx)
{
    return vp78_decode_init(avctx, IS_VP8);
}
