The given code slice appears to be a part of a Base64 decoding function. The line of code indicated as having a CWE-787 vulnerability is:

```c
ascii[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
```

CWE-787 refers to "Out-of-bounds Write" vulnerabilities. This type of vulnerability occurs when more data is written to a buffer than it is designed to hold, potentially causing the extra data to spill over into adjacent areas of memory.

In this specific case, the issue is likely due to the fact that `b64[1]` is being right-shifted by 4 bits (`b64[1] >> 4`), which will result in a value between 0 and 15 (since `b64[1]` is a `uint8_t`, which is an unsigned 8-bit integer). However, when this value is bitwise OR'd with the result of `b64[0] << 2`, the resulting value could potentially exceed the maximum value that can be stored in a `uint8_t` (255).

If `b64[0]` has a value of 240 or greater, the left shift operation `b64[0] << 2` will result in a value greater than 255, which cannot be represented by a `uint8_t`. When this value is then bitwise OR'd with the result of `b64[1] >> 4`, the resulting value will still be greater than 255.

However, the assignment `ascii[0] = ...` will truncate this value to fit within the `uint8_t` type, potentially causing an out-of-bounds write if the `ascii` buffer is not large enough to hold the truncated value.

To fix this vulnerability, you should ensure that the `ascii` buffer is large enough to hold the decoded values, and that the decoding operation does not attempt to write beyond the bounds of the buffer.

Here is an example of how you might modify the code to prevent this vulnerability:

```c
static inline void DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], uint8_t b64[B64_BLOCK])
{
    uint16_t temp = (uint16_t)(b64[0] << 2) | (b64[1] >> 4);
    if (temp > 255) {
        // Handle error: decoded value exceeds maximum value for uint8_t
    } else {
        ascii[0] = (uint8_t)temp;
    }
}
```

In this modified version, the decoding operation is performed using a `uint16_t` temporary variable, which can hold values up to 65535. The result is then checked to ensure it does not exceed the maximum value that can be represented by a `uint8_t` before being assigned to `ascii[0]`. If the decoded value does exceed this maximum, an error is handled accordingly.