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

The root cause of this vulnerability is that the `memmove` function at line 15 does not perform any bounds checking on the destination buffer `buf`. 

The `memmove` function is used to shift the contents of the buffer to the left by `start` bytes. However, if `start` is greater than or equal to `initial_read`, the expression `initial_read - start` will be zero or negative. 

In this case, the `memmove` function will still attempt to write to the buffer, but it will write past the end of the buffer, causing an out-of-bounds write. This can lead to undefined behavior, including crashes, data corruption, and potentially even code execution.

Additionally, if `start` is greater than `bufsize`, the `memmove` function will also write past the end of the buffer, even if `initial_read - start` is positive.

To fix this vulnerability, the code should add bounds checking to ensure that the `memmove` function does not write past the end of the buffer. For example:

```c
if (start < initial_read && start < bufsize) {
    size_t bytes_to_move = initial_read - start;
    if (bytes_to_move > bufsize - start) {
        bytes_to_move = bufsize - start;
    }
    memmove(buf, buf + start, bytes_to_move);
}
```

This code checks that `start` is less than `initial_read` and `bufsize` before attempting to move the data. It also calculates the number of bytes to move, ensuring that it does not exceed the remaining space in the buffer.