Q: Given the following code slice:
```
1 static void
2 bytes_chunk_extract (uintmax_t k, uintmax_t n, char *buf, size_t bufsize,
3                      size_t initial_read, off_t file_size)
5   off_t start;
10   start = (k - 1) * (file_size / n);
13   if (initial_read != SIZE_MAX || start < initial_read)
15       memmove (buf, buf + start, initial_read - start)```
which has a CWE-787 vulnerability at line:
```
15       memmove (buf, buf + start, initial_read - start);
```
Please analyze the root cause of the vulnerability.