Secure link: fixed stack buffer overflow.

Secure link: fixed stack buffer overflow.

When secure link checksum has length of 23 or 24 bytes, decoded base64 value
could occupy 17 or 18 bytes which is more than 16 bytes previously allocated
for it on stack.  The buffer overflow does not have any security implications
since only one local variable was corrupted and this variable was not used in
this case.

The fix is to increase buffer size up to 18 bytes.  Useless buffer size
initialization is removed as well.