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 int vp7_decode_block_coeffs_internal(VP56RangeCoder *r,
                                            int16_t block[16],
                                            uint8_t probs[16][3][NUM_DCT_TOKENS - 1],
                                            int i, uint8_t *token_prob,
                                            int16_t qmul[2],
                                            const uint8_t scan[16])
{
    return decode_block_coeffs_internal(r, block, probs, i,
                                        token_prob, qmul, scan, IS_VP7);
}

static int vp7_decode_block_coeffs_internal(VP56RangeCoder *r,
                                            int16_t block[16],
                                            uint8_t probs[16][3][NUM_DCT_TOKENS - 1],
                                            int i, uint8_t *token_prob,
                                            int16_t qmul[2],
                                            const uint8_t scan[16])
{
    return decode_block_coeffs_internal(r, block, probs, i,
                                        token_prob, qmul, scan, IS_VP7);
}
