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 vp7_decode_mv_mb_modes(AVCodecContext *avctx, VP8Frame *cur_frame,
                                   VP8Frame *prev_frame)
{
    vp78_decode_mv_mb_modes(avctx, cur_frame, prev_frame, IS_VP7);
}

static void vp7_decode_mv_mb_modes(AVCodecContext *avctx, VP8Frame *cur_frame,
                                   VP8Frame *prev_frame)
{
    vp78_decode_mv_mb_modes(avctx, cur_frame, prev_frame, IS_VP7);
}
