Step 1: analyze the CWE-787 (Out-of-bounds Write) vulnerability at line 3 of the DecodeBase64Block function, let's reason about the behavior step-by-step:

1. Input analysis:
   - The function takes two parameters: uint8_t ascii[ASCII_BLOCK] and uint8_t b64[B64_BLOCK]
   - ASCII_BLOCK and B64_BLOCK are not defined in the given code, but we can assume B64_BLOCK is 4 based on usage in the DecodeBase64 function

2. Vulnerability context:
   - The vulnerability is in the line: ascii[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
   - This operation writes to ascii[0] using values from b64[0] and b64[1]

3. Potential issues:
   - There's no bounds checking on the ascii array
   - The function assumes ascii has at least one element
   - There's no validation that b64 has at least two elements

4. Calling context:
   - DecodeBase64Block is called from DecodeBase64 function
   - Before calling, DecodeBase64 checks if dest_size < *decoded_bytes + numDecoded_blk
   - However, it doesn't ensure that ascii (which is dptr in DecodeBase64) has enough space

5. Vulnerability condition:
   - If ascii is passed with a size of 0, writing to ascii[0] would be an out-of-bounds write
   - This could happen if dest in DecodeBase64 is allocated with insufficient size

6. Consequences:
   - Writing outside the bounds of ascii could corrupt adjacent memory
   - This may lead to undefined behavior, crashes, or potential security exploits

Conclusion:
The vulnerability arises because DecodeBase64Block assumes ascii has at least one element without verifying it. If called with an ascii array of size 0, it would cause an out-of-bounds write. The calling function (DecodeBase64) checks overall buffer size but doesn't guarantee each call to DecodeBase64Block has sufficient space in the ascii parameter. To fix this, bounds checking should be added in DecodeBase64Block, or the calling function should ensure ascii always has at least ASCII_BLOCK elements.
Based on the analysis, here are five possible patches for the CWE-787 vulnerability:

Patch 1: Add bounds checking in DecodeBase64Block
```c
static inline void DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], uint8_t b64[B64_BLOCK], size_t ascii_size)
{
    if (ascii_size >= 1) {
        ascii[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
    }
    if (ascii_size >= 2) {
        ascii[1] = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
    }
    if (ascii_size >= 3) {
        ascii[2] = (uint8_t) (b64[2] << 6) | (b64[3]);
    }
}
```

Patch 2: Use a safer function that returns success/failure
```c
static inline bool DecodeBase64Block(uint8_t ascii[ASCII_BLOCK], uint8_t b64[B64_BLOCK], size_t ascii_size)
{
    if (ascii_size < ASCII_BLOCK) {
        return false;
    }
    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 true;
}
```

Patch 3: Use a macro to ensure ASCII_BLOCK size
```c
#define DECODE_BASE64_BLOCK(ascii, b64) do { \
    _Static_assert(sizeof(ascii) >= ASCII_BLOCK, "ascii buffer too small"); \
    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]); \
} while(0)
```

Patch 4: Use pointer arithmetic and bounds checking
```c
static inline void DecodeBase64Block(uint8_t *ascii, size_t ascii_size, const uint8_t *b64)
{
    if (ascii_size >= 1) {
        *ascii++ = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
    }
    if (ascii_size >= 2) {
        *ascii++ = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
    }
    if (ascii_size >= 3) {
        *ascii = (uint8_t) (b64[2] << 6) | (b64[3]);
    }
}
```

Patch 5: Use a struct to ensure proper sizing
```c
struct AsciiBlock {
    uint8_t data[ASCII_BLOCK];
};

static inline void DecodeBase64Block(struct AsciiBlock *ascii, const uint8_t b64[B64_BLOCK])
{
    ascii->data[0] = (uint8_t) (b64[0] << 2) | (b64[1] >> 4);
    ascii->data[1] = (uint8_t) (b64[1] << 4) | (b64[2] >> 2);
    ascii->data[2] = (uint8_t) (b64[2] << 6) | (b64[3]);
}
```

Each of these patches addresses the vulnerability in a different way, providing various levels of safety and compile-time checks. The choice of patch would depend on the specific requirements and constraints of the project.