The CWE-787 vulnerability is an "Out-of-bounds Write" vulnerability. 

The root cause of this vulnerability is that the `memcpy` function at line 12 does not perform any bounds checking on the destination buffer `r + i`. 

In this specific case, the loop starts at `i = 3` and doubles `i` in each iteration until `i` is no longer less than `size / 2`. However, the `memcpy` function is called with `i` as the size of the data to be copied, and the destination address is `r + i`. 

If `i` exceeds `size - i` (i.e., the remaining space in the buffer), the `memcpy` function will write beyond the end of the buffer, resulting in an out-of-bounds write. This can cause undefined behavior, including crashes, data corruption, or even code execution.

To fix this vulnerability, you should ensure that the destination buffer has enough space to hold the data being copied. You can do this by changing the loop condition to `i < size - i` to ensure that `i` never exceeds the remaining space in the buffer.

Here's the corrected code:

```c
static void
fillpattern (int type, unsigned char *r, size_t size)
{
    size_t i;
    for (i = 3; i < size - i; i *= 2)
        memcpy (r + i, r, i);
}
```

Alternatively, you can also use `memmove` instead of `memcpy` to handle overlapping source and destination buffers. However, in this specific case, using `memmove` would not fix the out-of-bounds write vulnerability.