X25519: Check for all zeros value

As per RFC 7748:
```
Both now share K = X25519(a, X25519(b, 9)) = X25519(b, X25519(a, 9))
as a shared secret. Both MAY check, without leaking extra
information about the value of K, whether K is the all-zero value and
abort if so (see below). Alice and Bob can then use a key-derivation
function that includes K, K_A, and K_B to derive a symmetric key.
```

Just to double check if we do that

Locate the Go X25519 implementation and inspect how the shared secret is handled, using the RFC 7748 excerpt in the issue as the reference. Confirm whether an all-zero result is checked without leaking additional information, then document the finding or identify the missing behavior.

Written by the indexing model from the issue text.