Here are five possible code completions for the given function, addressing the CWE-787 vulnerability:

1. Add bounds checking:

    if (ASCII_BLOCK >= 3 && B64_BLOCK >= 4) {
        ascii[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
        ascii[1] = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
        ascii[2] = (uint8_t) (b64[2] << 6) | (b64[3]);
    }

2. Use a safer alternative function:

    size_t output_length;
    if (base64_decode(ascii, ASCII_BLOCK, b64, B64_BLOCK, &output_length) == 0) {
        // Decoding successful
    } else {
        // Handle error
    }

3. Implement input validation:

    if (ValidateBase64Input(b64, B64_BLOCK)) {
        ascii[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
        ascii[1] = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
        ascii[2] = (uint8_t) (b64[2] << 6) | (b64[3]);
    } else {
        // Handle invalid input
    }

4. Use a fixed-size buffer with bounds checking:

    uint8_t temp[3];
    temp[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
    temp[1] = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
    temp[2] = (uint8_t) (b64[2] << 6) | (b64[3]);
    memcpy(ascii, temp, MIN(ASCII_BLOCK, 3));

5. Implement error handling:

    if (ASCII_BLOCK < 3 || B64_BLOCK < 4) {
        // Handle error: insufficient buffer size
        return;
    }
    ascii[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
    ascii[1] = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
    ascii[2] = (uint8_t) (b64[2] << 6) | (b64[3]);