Based on the provided code snippet and the specified vulnerability (CWE-787: Out-of-bounds Write) around line 69, here are five possible patches to address the issue:

Patch 1:
```c
68             leading_sp = 0;
69             memset(b64, 0, sizeof(b64));
70         }
```
This patch ensures that we're using the array name directly in memset, which is safer than using the address-of operator.

Patch 2:
```c
68             leading_sp = 0;
69             for (int i = 0; i < B64_BLOCK; i++) {
70                 b64[i] = 0;
71             }
72         }
```
This patch replaces memset with a manual loop to reset the b64 array, ensuring we don't write beyond its bounds.

Patch 3:
```c
68             leading_sp = 0;
69             memset(b64, 0, B64_BLOCK * sizeof(uint8_t));
70         }
```
This patch explicitly specifies the size of the b64 array in bytes, reducing the risk of overflow.

Patch 4:
```c
68             leading_sp = 0;
69             if (sizeof(b64) == B64_BLOCK * sizeof(uint8_t)) {
70                 memset(b64, 0, sizeof(b64));
71             } else {
72                 for (int i = 0; i < B64_BLOCK; i++) {
73                     b64[i] = 0;
74                 }
75             }
76         }
```
This patch adds a safety check to ensure the size of b64 matches the expected size before using memset, falling back to a manual loop if not.

Patch 5:
```c
68             leading_sp = 0;
69             secure_memzero(b64, sizeof(b64));
70         }
```
This patch replaces memset with a hypothetical secure_memzero function, which is designed to safely zero out memory without risk of optimization-related issues or overflows.

Note: The secure_memzero function would need to be implemented elsewhere in the codebase or included from a security library.