On line 77:
strncat(strncpy(&buf, path, sizeof(buf)), basename(url), sizeof(buf));
As stated in strncat (3):
If src contains n or more bytes, strncat() writes n+1 bytes to dest (n from src plus the terminating null byte). Therefore, the size of dest must be at least strlen(dest)+n+1
if strlen(basename) + strlen(path) + 1 > sizeof(buf), buffer overflow occurs.
On line 88:
n = BIO_read(resp.bio, buf, BUFSIZ);
It read at most BUFSIZ but sizeof(buf) is PATH_MAX (line 71).
It is very unlickely they will happend due to the current value of PATH_MAX on Linux.