Step 1:  root cause of the CWE-787 (Out-of-bounds Write) vulnerability in the given code slice is:

The function DecodeBase64Block is performing a base64 decoding operation without proper bounds checking on the input or output arrays. Specifically:

1. There's no validation of the size or contents of the input array b64.
2. There's no check to ensure that ascii[0] is a valid index within the ascii array.

The vulnerability occurs because:

1. If b64[0] or b64[1] contain unexpected values, it could lead to writing an unintended value to ascii[0].
2. If the ascii array is smaller than expected or not properly allocated, writing to ascii[0] could result in an out-of-bounds write.

To mitigate this vulnerability:

1. Implement input validation to ensure b64 contains valid base64 characters.
2. Add bounds checking to verify that ascii has sufficient space for the decoded output.
3. Consider using a safer decoding function that includes built-in bounds checking and error handling.

By addressing these issues, the risk of out-of-bounds writes can be significantly reduced, improving the overall security and reliability of the code.
Based on the root cause analysis, here are five possible patches to address the CWE-787 vulnerability in the given code:

Patch 1: Add input validation and bounds checking
```c
static inline int DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], uint8_t b64[B64_BLOCK], size_t ascii_size, size_t b64_size)
{
    if (ascii_size < 3 || b64_size < 4) {
        return -1; // Insufficient buffer size
    }
    
    // Validate input characters
    for (int i = 0; i < 4; i++) {
        if (b64[i] >= 64) {
            return -1; // Invalid base64 character
        }
    }

    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]);
    
    return 0; // Success
}
```

Patch 2: Use a safer decoding function with built-in checks
```c
#include <openssl/bio.h>
#include <openssl/evp.h>

static inline int DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], uint8_t b64[B64_BLOCK], size_t ascii_size)
{
    BIO *b64_bio, *mem_bio;
    int decoded_size;

    b64_bio = BIO_new(BIO_f_base64());
    mem_bio = BIO_new_mem_buf(b64, B64_BLOCK);
    mem_bio = BIO_push(b64_bio, mem_bio);

    decoded_size = BIO_read(mem_bio, ascii, ascii_size);

    BIO_free_all(mem_bio);

    return (decoded_size > 0) ? 0 : -1;
}
```

Patch 3: Implement a custom safe decoding function
```c
static inline int DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], const uint8_t b64[B64_BLOCK], size_t ascii_size)
{
    static const uint8_t d[] = {
        66,66,66,66,66,66,66,66,66,66,64,66,66,66,66,66,66,66,66,66,66,66,66,66,66,
        66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,62,66,66,66,63,52,53,
        54,55,56,57,58,59,60,61,66,66,66,65,66,66,66, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
        10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,66,66,66,66,66,66,26,27,28,
        29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,66,66,
        66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,
        66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,
        66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,
        66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,
        66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,66,
        66,66,66,66,66,66
    };

    if (ascii_size < 3) return -1;

    uint8_t i0 = d[b64[0]], i1 = d[b64[1]], i2 = d[b64[2]], i3 = d[b64[3]];
    if (i0 > 63 || i1 > 63 || i2 > 63 || i3 > 63) return -1;

    ascii[0] = (uint8_t) (i0 << 2) | (i1 >> 4);
    ascii[1] = (uint8_t) (i1 << 4) | (i2 >> 2);
    ascii[2] = (uint8_t) (i2 << 6) | i3;

    return 0;
}
```

Patch 4: Use a macro for bounds checking
```c
#define SAFE_WRITE(dest, src, dest_size, index) \
    do { \
        if ((index) < (dest_size)) { \
            (dest)[(index)] = (src); \
        } else { \
            return -1; \
        } \
    } while(0)

static inline int DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], uint8_t b64[B64_BLOCK], size_t ascii_size)
{
    SAFE_WRITE(ascii, (uint8_t) (b64[0] << 2) | (b64[1] >> 4), ascii_size, 0);
    SAFE_WRITE(ascii, (uint8_t) (b64[1] << 4) | (b64[2] >> 2), ascii_size, 1);
    SAFE_WRITE(ascii, (uint8_t) (b64[2] << 6) | (b64[3]), ascii_size, 2);
    return 0;
}
```

Patch 5: Use a struct with size information
```c
typedef struct {
    uint8_t *data;
    size_t size;
} SafeBuffer;

static inline int DecodeBase64Block(SafeBuffer *ascii, SafeBuffer *b64)
{
    if (ascii->size < 3 || b64->size < 4) {
        return -1;
    }

    ascii->data[0] = (uint8_t) (b64->data[0] << 2) | (b64->data[1] >> 4);
    ascii->data[1] = (uint8_t) (b64->data[1] << 4) | (b64->data[2] >> 2);
    ascii->data[2] = (uint8_t) (b64->data[2] << 6) | (b64->data[3]);

    return 0;
}
```

Each of these patches addresses the vulnerability in a different way, providing various levels of safety and complexity. The choice of which patch to use would depend on the specific requirements and constraints of the project.