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 decode_block_coeffs(VP56RangeCoder *c, int16_t block[16],
                        uint8_t probs[16][3][NUM_DCT_TOKENS - 1],
                        int i, int zero_nhood, int16_t qmul[2],
                        const uint8_t scan[16], int vp7)
{
    uint8_t *token_prob = probs[i][zero_nhood];
    if (!vp56_rac_get_prob_branchy(c, token_prob[0]))   // DCT_EOB
        return 0;
    return vp7 ? vp7_decode_block_coeffs_internal(c, block, probs, i,
                                                  token_prob, qmul, scan)
               : vp8_decode_block_coeffs_internal(c, block, probs, i,
                                                  token_prob, qmul);
}

int decode_block_coeffs(VP56RangeCoder *c, int16_t block[16],
                        uint8_t probs[16][3][NUM_DCT_TOKENS - 1],
                        int i, int zero_nhood, int16_t qmul[2],
                        const uint8_t scan[16], int vp7)
{
    uint8_t *token_prob = probs[i][zero_nhood];
    if (!vp56_rac_get_prob_branchy(c, token_prob[0]))   // DCT_EOB
        return 0;
    return vp7 ? vp7_decode_block_coeffs_internal(c, block, probs, i,
                                                  token_prob, qmul, scan)
               : vp8_decode_block_coeffs_internal(c, block, probs, i,
                                                  token_prob, qmul);
}
